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I found these on the left side of the transmission. These are the only stamps I have been able to find on the transmission, are there others? The car is a 400ci 4 speed manual. I have found other stamps on the motor and on the carburetor confirming they are numbers matching but I haven't had any luck deciphering these numbers. I know they are very hard to read

From what I can tell it says:

8794 5M18 4

There are a few numbers that come before the 8794 that are near impossible to tell what they say.
 

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The pics you are showing are the part number and date stamped into a 10MT series starter. An original '71 400 4bbl or 455 Dport engine starter will have part number that ends in "8446" = 1108446.

8794 5M18 4

the starter in the pic, has a larger, later part number, and in decyphering a Dec 18th of 75 assembled date.

To ID your transmission, assuming its the original Muncie, will need to jack the car up, or put on a lift, and look at the passenger side of the case. The last 3 digits of the main case casting should be "661". There will be Muncie dated assembly ID code, like P1M17A, and a partial VIN stamped on the main case, that is if the transmission case is original, and not a counter replacement case.

both of these stampings are usually on the passenger side of the later Muncie case on the raised 1" wide area at the rear of the case where the tail housing bolts on. Occasionally, the partial VIN will not be stamped next to the P code, but on top of the case close to the tailhousing flange. One thing many forgers dont realize is there were several different fonts used in the gangstamprs that stamped the partial VIN stampings and the font depended on which original assembly plant.

On the above example Muncie code P1M17A, it decodes as
P= Muncie
1= '71 production
M= August, in this case, August of '70
17= day of month
A = M20 wide ratio

B & C were also characters used to ID the Muncie gear set.
B for M21, was not used in '71 or '72 Pontiacs
C was the ID code for M22, only avail behind a '71 400 4bbl with special order axle ratio (3.90), otherwise the 400 4bbl '71 Poniac A body with a Muncie would get the M20.
The M22 was far more common behind the '71 & 72 455 HO engines.

Feel free and post your Muncie code and if an original '71 or '72 Muncie, can forward to you where that particular date "fits" in the production year, by original usage at various assembly plants.
 

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Incredibly helpful information

I feel pretty dumb posting the pictures of my starter:( I was able to find all the numbers you guided me to and you were right on with everything. The easy part was finding the large raised numbers that has the 3925661 but after some searching and cleaning off more grease I found the other codes directly left of this that were stamped: P1C08A and it wasn't until I took the picture that I saw the VIN stamp that is a bit difficult to make out in the picture but it matches my VIN having 21Z122081. Thanks so much for your help! It feels great knowing that the engine and transmission are numbers matching.
 

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