The Reverb in early Superfly amps wasn't the greatest. The design was very unique, and it needed a little extra time for it to be right. Unfortunately, we shipped amplifiers before that time. Since then, we have improved the circuit and upgraded many of the original amplifiers for better reverb performance.
The box you have sounds like it was made by me. I bet those RCA phono jacks ARE 40 years old (Still better than new chinese parts). Yes, all that box does is mute some high frequency response from the reverb pan. It's a patch, not really a fix. But, corrosion on the inside connector could be causing the signal to cut out, so you may want to have a closer look at this, and/or try the reverb without this box (will be brighter).
The pan can be reversed (input for output), and the reverb will be less, but it should still work. If it doesn't , then the side plugged into the RED wire (drive wire from the chassis) is broken in the pan.
The amp SHOULD have a serial number. There should be a back panel serial number, plus one on the inside of the chassis near the power transformer, plus another one, either underneath the chassis, or underneath the front panel.
There was one amplifier shipped to the UK that had the reverb re-worked by Adrian (Orange amp designer), that would have been #0036, I believe. Supposedly, that one was "Fixed".
If the reverb is cutting out, I would check the cables and the reverb pan first. These are the first two things to fail in any amp (Those and the tubes). The Reverb function has 8 clicks, from no reverb to maximum reverb.
Cutting back the sensitivity on the reverb is fairly easy. One capacitor is removed from the chassis to reduce the amplification gain. The chassis needs to be removed, but the work can be done by the owner with simple hand tools.
TUBES- Small tube near the output transformer is a 12AT7, this is for the Reverb, only. All other small tubes are the 12AX7 type. The middle 12AX7 is use partially for Reverb, the other half is the Global tone control driver. All other tubes are guitar preamp or power amp driver. Schematic- We don't provide schematics, but we can provide detail on particular parts of the amplifier to aid you in the repair/restoration. Right now, check the cables and pan, and let me know what you find.
Let me know if you need further assistance,
The response from Bruce (opposite) was helpful and informative, although his description of where the reverb driver tube should be ("near the output transformer") was a bit ambiguous - near the speaker output transformer or the reverb output/send transformer?
By much trial and error I later figured it should be the latter.
Over the next few days I did my best to check Bruce's points as well as I could and then emailed him again - see part 5.