Given the timeframe, location, and the grille border trim, they may well be the AC568 circuit, which has a number of "interesting" changes made by CBS engineers to "improve" the amps, such as the funky hybrid fixed/cathode bias with individual 150 ohm resistors per 6L6 and nonpolarized electrolytic caps tying them together, but just for the AC portion of the signal above a certain frequency, and a number of other things, a fair number of which got rolled back in the 1969 amps. Not sure if these AC568s were the beginning of the plastic-insulated stranded wire that could not be run under the circuit board and were usually poorly dressed, which necessitated snubber caps to squelch oscillation, but the crap wire and snubber caps mostly stayed after the 1969 circuit update....