The 7A13 is a differential comparator with +-10V internal offset. So you can display a 10V DC signal with 1 mV/div resolution.
Those now too are on my list.
Small correction however, you said 7804 while mine is 7704A
Keep in mind that the 7A13 is full of very Tek specific relays. With a footprint that prevents an easy replacement. Unfortunately they do fail on a regular basis, specially the double contact ones. In the old days they actually repaired them, using a Dremel to open them. I made two test jigs to test them. Seems to help if you connect them to a FG and let them switch a large number of times.
Tek 7k stuff is addictive. On the bench: 7603 with 7A22, 7A13 and 7B53A. 7623A with 7L5+TG. On cart: 7704A with 7A18, 7S11, 7T11 and 7B70. Then 5 more 7xxx mainframes as spare and a number of parts/fix-up plugins. Like another 7L5 (looking for the big knob assembly), 7A13 without relays (guess why), some more 7A..amps and a number of spare 7B53As. And one self made 7xxx extender cable. About half the mainframes have new PS caps, using adapter pcb's. I had to fix the 7704A a number of times. Bad resistors/arcing in the HV section. Primary bridge rectifier shortened and a shortened tant cap somewhere. Keeps you busy. But all easy fixes.
I have a similar list of TM5xx(x) stuff, next to the regular gear. Yep, I fit in here