Nice find. We just made one batch of those, really just sort of "for fun". Pretty cool amp. Use an A/B box to switch channels.
The power switch on the rear works like this.
100W = All four tubes on at 500VDC
50W = Two power tubes on at 500VDC
25W = All four tubes on at 250VDC
10W = Two power tubes on at 250VDC.