Look on the right side of the 4-speeds and see if there is a code tag. If so, look for a 5359389 to indicate a T-177, or a 5359390 for a T-176. Quite a few old men like me are interested in the T-177, though I have two already.
Engines are a bit tougher. If you are really interested:
The 70 and 71 GEN-3 Tall Decks produced more HP, so they are of interest to some. I have a spare engine that is a GEN-2 290 Short Deck that produces more HP than a 72-up 304. The big difference was the compression ratio. There are other ways to increase the compression ration, but some purists like the old engines that were designed that way.
As us "Old Men" get older, we look for these little things from our youth, but it is a select market. Most of us have acquired what we need, and just struggle to keep what we have going.