The amp should have a serial number inside the chassis, near the input jack even if the back panel serial number was removed.
Might also be one near the on/off switches, inside the chassis.
You've got an intermittent reverb. This could be easily caused by a cable that is on the blink. Most all the internal cables can be unplugged and checked out individually. Many of those cables are crimped connector types, which can be pulled apart, stripped, tinned, then crimped again and then soldered. That won't fail, but the crimped only connectors can.
So I've got an intermittent reverb (paragraph 2)? I think I knew that, as well as I knew that it didn't for some reason have a serial number.
At this point I tried to steer back toward the idea of getting the amp repaired - see part 16.