Very early (November 1978) mark 2a with the 60/100 option.
when in 60 watt, which 2 power tubes are "on"?
Also...On the bottom of the chassis in between tube sockets there is a pre amp out and power amp in jacks (again, on the bottom, not on the back), however, these are not labeled. So if I am looking at the bottom of the chassis with the back of the chassis closest to me, which is th pre amp tap and which is the power amp?
Same with the Reverb RCAs no "Send" and "return" on them. Looking at the bottom of the chassis with the back closest to me, is the closer rca jack send or return?
I can't answer most of your questions directly but below is a way to figure out what is what.
1. The tubes that are on when in 60 watt mode are the outside tubes.
2. Refer to the schematic at the link below for the pre-amp-out/power-amp-in question. I am assuming the schematic labeled Mark II is the same as the Mark IIA.http://www.webphix.com/schematic%20heav ... oogie.html
Looking at the schematic the pre-amp-out/power-amp-in are labeled as the same jack. There is another jack labeled as Effects Send. You should be able to identify the Effects Send by plugging in a cord. With a cord plugged in the shorting connection will open and disable the output, i.e. no amplification. A cord into the pre-amp-out/power-amp-in will not affect output as there is not a shorting connection that opens with a cord in place.
3. The only way I can think of to id the two reverb RCA connectors without going inside the amp is as follows. You will need an ohm meter. With the amp OFF and unplugged measure the resistance of one of the RCA jacks. It will either be a real low resistance (this is the one connected to the transformer or reverb input) or about 220K (which is the reverb output). Sometimes the ground side of the RCA connectors are not connected so use the chassis for the ground rather than the RCA jack. If you are inside the amp then just trace the circuit out.
Clearly to do the above you need to be able to read a schematic and have access to an ohm meter. Hope this helps and let me know if the above explanation is not clear or you need clarification.