[COPO]

Just took a look at your previous post for the car and the link to the photos. Seems like a very nice car and very well optioned. My first car that I bought in 1980 back in high school was a 70 Z-28 which I still own 33 years later, so I am partial to these cars. Mine is very similarly equipped to this one you are looking at and has a "28.10" stamping on the TT.

Even if the trans stamping is correct as the seller conveyed to you, (April 20) I think it is close enough to the body build date that it could very well be original to the car. The second week of April could be as late as Friday April 11th, so that would be 9-days prior to the transmission assembly date, so while somewhat unusual would not be out of the realm of possibilities.

Good luck.