The Roundie from Hell (RFH) used a single-stage 6SQ7 triode for the AF amp. I breadboarded your mod for the RFH and liked the results -- except it still didn't have a good signal to amplify better, so I returned it to stock. The RFH I think went to a technician more skilled than I and who knows where it ended up...
According to its Sams, the combo version of the RFH uses a 6SJ7 pentode for the AF amp, and uses the 6SQ7 as a phase inverter for the push-pull 6K6's. This one doesn't have the 6SJ7/6SQ7 combination -- instead it looks like the 6SC7 dual-triode is pulling double-duty -- 1/2 of it is the AF amp and the other half is the phase inverter. So I guess the 6SC7 belongs. This one is definitely a one-off from the RFH, 20B1. Still looks like "**C1" but the first 2 digits are unreadable.