On our way to a convoluted evolution...

From the simple wooden planes that made the basic joinery possible: Rabbet, Plough, Fillester, Dado, Tonguing and Grooving planes, to its perhaps foregone conclusion.

What the... Am I talking about? Believe it or not, I am simply answering Matt question about that odd shaped metal piece I called a Stanley No 45 Cam rest.
Yeah really!! :-)

In the beginning it was simple, the basic joinery in Western woodworking had evolved into a set of easily executed joinery with a chisel and a saw.
But being repetitive and mostly made up of square shoulders, they could easily be made with some sort of wooden planes.

GROOVY PLANES

A simple chisel stuck in a piece of 2X4 construction wood, could serve as a router plane, a plough plane etc depending how installed and used.

The Dado, the Groove and the Rabbet only differ in their orientation to the wood grain and the open edge of the rabbet.

Lets cut the rabbet first, it is the easiest.
Only thing our plane need is a bottom sole the width of the rabbet and a blade whose cutting edge is the full width of the sole.

That would required a set of planes of various rabbet width, as many as required.

Make the sole as wide as the largest rabbet you want to cut and install a fence riding under the sole. That would only expose a variable part of the cutting edge, making variable width rabbets.
We have a Filletster plane.

Top - Stanley No 78 Duplex Rabbet Filletster plane. 

Called Duplex since it can also be converted into a bull nose plane.

Bottom - Stanley No 191 Rabbet plane

To make a groove, we only have to worry about having a cutter the width of the groove and have a fence to set the groove a set distance off the reference edge.

That would required a set of planes of various groove width, as many as required. And would required a set of planes of various fence spacing width, as many as required.

Make the fence variable and make the cutter replaceable by cutters of various width and we have a winner! Only one plane with a set of 8 cutters:
The Plough (Plow in US) plane

Greenfield Tool Co No 52 screw arm plough plane

The cutter centering is accomplished by a V groove in the cutter back registering with the plane skate, which is essentially the plane sole. We don't need a large skate (sole) it would be riding at the bottom of the groove, and these are never very wide, they are grooves...

MATCHING plane
Or Tongue & Groove

To cut the groove requires a simple fixed blade plow plane and a matching
plane to cut the corresponding tongue.

If you go the wooden plane way, make very sure that the so called match set of planes you are buying are in fact a match set... just saying DAMHIKT

To me the best plane for cutting a match set of T&G remained

 the Stanley No 48 with its swinging fence.

So to the very astute would be plane inventor in the late 19th century, a logical conclusion could have been something like:
Need a body to hold the replaceable cutter(s) square to the face and adjustable in depth. Only need a skate for a sole, but for added stability when adjusting wide, a second movable skate should be added.
Make the fence adjustable under the sole for a Fillister effect.
Add a few simple moulding profile and you add to its repertoire.

Drum roll please... Enter the dreaded and loved Combination plane. A marvel of engineering and frustration....

Top - Stanley No 45

Bottom - Siegley No 2

In  the days priors such mechanical contraptions, workers also used separate moulding planes, in order to tackle a variety of profiles...
That meant carrying an assortment of moulding planes inside a toolbox.
Early on these wooden moulding planes were not of a standard length, varying from approx 9 to 10 inches long, some slightly longer even. For ease of portability and storage, many of these early 18th century planes were later docked to fit the ''newly introduced standard'' of 9 inch.

The challenge for would be planemakers seeking fame and fortune, was to come up with a single plane which could replace a box load of wooden moulding planes.

Yes, the combination plane can easily replaced all the above mentioned planes in various sizes and fenced setting, add a few more accessories and you can inflict even more frustrations to the unwary.... Masochists :-)

One of the first successful design, later to be acquired by Stanley, was the Miller patent.  In truth it was not quite a combination plane as we know it today, but it was the one that start it all.

The CH Miller Improved Carpenter's plane

Charles H Miller of Brattleborough Vermont received a patent on 28 June 1870 for what he called an "Improved carpenter'' plane.
What was the improvement you asked? It was the combination of 3 in 1 tools, It's a grooving, rabbeting and a smooth plane. Essentially it is a plow plane which could work as a smoothing plane with a large blade supported by an attached side mounted bed (called the Filletster bed) to support the blade. That smoother feature is greatly exaggerated, it would make a lousy smoother...

Once the patent and its designer were acquired by Stanley, they experimented with various design of metallic combination and plow plane, the No 41, 42, 43 and 44 planes, before settling on the No 45 and later taking the design to its ultimate gizmo centric No 55. A planing mill in itself!

No 41 is a cast iron Filletster, Plow and Matching plane
No 42 is a gunmetal Filletster, Plow and Matching plane
No 43 is a cast iron  Plow and Matching plane
No 44 is a gunmetal Plow and Matching plane

No 46 and 47 are cast iron skew cutters versions
No 46 is the Traut's patent Plow, Dado, Fillister and Match plane came with 10 to 12 cutters
No 47 is same as No 46 (same casting useds) but came with less cutters, had 5 to 7 cutters

Because essentially such combination planes do not have a sole matching the cutter profile, some cutters would requires special sole bottom attachments such as Hollows and Rounds.  The skate idea even with a cam rest would not work on these profile cutters for their intended purpose.

The basic problem with the narrow skate used as a sole is one of stability, it becomes oh so easy to tip the plane. The more the fence is extended from the plane narrow body, or more apparent when using profiled or wider cutters, the more tippy it becomes and it is oh so easy to buggered up the profile of our cuts.

Solutions are two fold: add one or more skate to the plane.
Make the added skate(s) adjustable in both position and height and we have reached the zenith: the Stanley No 55

Or.... on our lowly No 45, the problem was addressed simply by adding a cam rest. Make the pivot hole off centered and you can adjust it for various height.

It first started to show up around 1902 with the Type 8 plane. The first incarnation was fragile and harder to manufactured so it was quickly changed to the type I found.

Pic from Ebay- Earlier short lived version

Note that both type share the same patent No of July 9th 1901

Original patent assigned to Justin Traut, 

Stanley's prolific plane patents dude

Cam rest installed between sliding section and fence 

for added support

That work, but in use it also act as a secondary depth stop.  And like any adjustments which are locked simply by a lone set screw on a round post does....
it tend to move, making a lousy depth stop :-)

You cannot stop progress.... Even if it is a very convoluted way to get there...

So how do you attach this after thought, it was patented in 1901 long after the No 45 was manufactured since 1884.

It goes between the auxiliary skate and the plane body, or between the fence and auxiliary skate (sliding section) so....
The fence then the auxiliary skate comes off...

So there you go Matt the 'simple' answer to your question: 

can you tell me what the cam is /does? :-)

Using the No 45, by Bill Anderson

Bob, supplier of convoluted thoughts since 1956