Hello folks;
I've been going through all the numbers on my drivetrain to see if things are matching, or from something else. It seems that everything is as it should be, besides the transmission. Please forgive any incorrect lingo as this is my first restoration and may be a little over my head. The car was meant to come with a M21 (according to the company I bought it off,although they turned out to be quite shifty), although my engine suffix is JA. However the transmission's Vin does not match my Vin, therefore I have no way of knowing what it is without some experienced advisors. Oh yeah, the car is a 1969 SS. Here are the numbers I've found:
Vin:217Z132275
Almost covered on the maincase is H1E12B
The main case has 3987945 ( the 5 is hard to read and may be an 8 or 6?)
The side cover, has 3952646
The tailshaft has 3911942
On the opposite side of the number above is a 6 with a line along the base of it.

Also, if it makes any help, the belhousing's part number is 3899621.

suggest that this post be moved to "Tag Team"

Well, I haven't figured out the VIN or the maincase (the VIN might be Pontiac since it starts with 2), but the other numbers indicate a 1971 Muncie heavy duty 3 speed. The production number, side cover, and extension housing all match information I found in Chevrolet By The Numbers. I couldn't find that maincase casting number, but it is a later number, and doesn't appear to match any Muncie, Saginaw, or BW numbers made through 1975. Maybe I missed something?
Bruce

Edit: The 621 bell housing is a late 1966 and later unit used with the 168 tooth flywheel. It is correct for a 1969 SS.

With these older transmissions, the case could have any gear set in it, no matter what the case or the input shaft indicates. Here's a sure fire method. Take the transmission out and count the number of times the output shaft rotates for each rotation of the input shaft for each gear. Could it be a three speed? If the car has a shifter installed, that should be an easy one to figure out.

No Bruce, you didn't miss anything. Those numbers don't match with anything that I am familiar with either. If it is in fact a HD 3sp, I'm not familiar with any of those numbers at all.
But, for sure, I don't recognize anything there that relates to a 4sp.

At least you have a desirable bellhousing.

I checked again, and the maincase number is 3987948, made a mistake before. As well, I switched two numbers in the vin, it's meant to be 271z132275. One thing I noticed this time, was that the date codes on the various casted parts are all different months. The main case is C251(4 digit date code?), the tailshaft is B41 and the sidecover is D291. Well, this transmission thing sorta puts a damper on things dont it? Is a HD 3spd identical to a M21 on the outside? I'm gonna go and take the side cover off to make sure of the production number, and while i'm at it , I guess taking a look inside wouldn't hurt either!. The car is well......thanks to me anyway totally disassembled, every nut and bolt, and the shifter is packed away on one of the many boxes (maybe not the best idea). I'll post back what I find, thanks for your help!

Yeah, H1E12B is correct, and some bolts have Rockford casted on the heads. Who would take out the original transmission, leave everything else and stick a 3-speed in place of it... looks like its time to get a new transmission isn't it?