This amp is totally re-done, with a new circuit built on a breadboard. The work is rumored to have been done in the '80s at S.I.R. in LA, but we have no way to confirm that. The work is tidy and professional, and consistent with the other supposedly S.I.R. modded Marshall we currently have for sale (even if it wasn't S.I.R., it's the same person who modded both amps). The new components have been painted with the values scratched off. The circuit is essentially Marshall, but with a more Soldano style extra triode gain stage (post EQ as opposed to pre-EQ), and pre-phase inverter master volume circuit. It also has a preamp line out. The transformers are original.
The extra knob in the input controls the added gain stage, the unmarked knob immediately to the left is the standard gain control, and the unmarked knob to the left of that is the master volume. The amp sounds great, and has that awesome modded '80s sound. Lots of tones available by mixing the two gain knobs in different positions. Recently inspected and given a clean bill of health at Amphead here in Portland. READY TO ROCK.