OK, I know the codes for M20, M21, M22 (A, B, C) were not supposed to start until around late October 1968, at least on Chevy cars. But could Pontiac have started sooner?

There is a Muncie for sale with a date stamp that has a C at the end that is earlier than when that C code was supposed to have started. I have one that looks almost exactly the same.
I believe the picture attached and the one that I have are from 1968 Pontiac cars - based on the partial VIN starting with 28. My transmission has M20 gears in it now, but could it have originally been an M22 or was someone maybe restamping them?

See attached image.

Is someone restamping them? I would guess they are for multiple reasons.

Real M22 4-speeds are quite rare. They did not come standard in very many car, only those with the very highest power.
They have a deservedly good reputation for being stronger.
Because of the above, they are high priced.

People have been advertising "M22" Muncies for many years. Most of those people didn't know how to tell one from the other so they just made the claim.
Some knew about the suffix stamp so they stamped the case and hoped the buyer would not know how to tell the internal difference.

It is not impossible but (I think) not likely that M22 gears were replaced with "standard" gears in these transmissions. I suppose if someone had their correct transmission and had an M22 they might switch gear sets to make their's better.

Did Pontiac start stamping suffix codes earlier than Chevrolet? Unknown to me. It is possible but the transmissions were all built in the same plant. When they started stamping them I suspect they all got stamped. Assembly workers didn't know where that transmission was going at that point.

Having said all that, no telling what has happened to these transmission over the last 40 years.

That doesn't even look like a "C" to me at the end, so I'm thinking anything that is stamped there, is after the tranny left Muncie. Personally, I don't know of ANY Pontiac that used an M22 until 1971.... I don't think you could even get one behind the 70 455HO...

I agree that it doesn't look like a c, more like an incomplete square with one side missing, considering how sharp the corners are, rather than being rounded. I've heard of shift indicators (day shift or night shift) on some Muncies, but I think that would be a D or N. Does the transmission have a metal tag attached by one of the side cover bolts?

Here are some pictuers of a real M-22, note the 2 plugs.

OK, I did a little more checking and found another trasmission stamped similarly that was probably from an Olds 442. My transmission stamp looks almost exactly the same. I can't take a picture right now because I can't find the digital camera. It does not matter anyway because the transmission I have has an M20 gearset in it now anyway.

But here is a picture of that 442 transmission.

I also asked on one of the Pontiac performance sites about that late October 1968 and if anyone knew of Pontiac starting the A, B and C codes sooner than Chevy. No answers yet.

I did some checking.

I am thinking that the small "c" may designate it as a non-Chevy transmission. I have attached a picture here of the one I have - from an early 1968 Van Nuys Firebird.

Does anyone else have ideas on this? It just seems odd to me that there are several transmissions out there like this with a similar small "c".

Well the small c does NOT designate it as a non-Chevy transmission. Apparently quite a few Camaros have the small c at the end of the date code.

Does anyone know anyone that worked at the Muncie plant in early 1968?

I am still stumped on this one. Anybody have ideas?

JMHO, the small c looks like a partial F. What do I know, I'm just a dumb mechanic.

The small c was only stamped onto 1968 Muncie transmissions. I have about 16-17 examples in my photo archive. The small c was stamped regardless of ratio.

Right. I have only seen this on 68 Muncies too. Any idea why it is there?

That looks like the bottom half of a E to me.

Its not a true C stamp like you would see on a 69 and newer M22. Its just a character than looks somewhat like a small c. I just want to know if anyone know if it means anything or why it is there.

The best answer I have been able to find is that the small c is an inspection stamp.