Decoding Numbers on Muncie Trans

[Vince]

I've been the owner of a 1969 Z/28 RS for 12 years now but only recently began checking the numbers and coding in great detail.  I have been able to decode my VIN, trim tag, protect-o-plate, chassis broadcast sheet, engine stamping, and rear end stamping.  They all point to the car being a real Z with the RS option, original engine, and original rearend.  I'm sure the tranny is not original though as I could find no number on it that matched the one on the POP. 

Here are the numbers I was able to get of the passenger side of the trans:

3857584 GM
3885, then either a O, C, or G, followed by 10
3858403
P0207

Thanks guys for your help.  I enjoy this website, read it almost daily, and have learned a lot about Camaros from the experts on here. 

[KurtS]

3885010 is a 66-67 casting.
The date format is 66, Feb 7th.

So, no, it's not original.

[Vince]

Thanks for the info, Kurt.  Is there any way to tell externally whether my trans is a M20, M21, or M22?   

[KurtS]

# of grooves on the input shaft and whether it has a drain plug or not.