Changed from EL 34 to KT66 and a bias pot was added and a master volume, removed? (Plug in the Mains punchout)
What we think they were doing is trying to get the KT66 tone with the rectifier tube like Clapton's
The following pictures will show that this was not too invasive a procedure and could be reversed.
This is a 220 amp despite the polarity switch and the power transformer has the protective coating from the Marshall factory
This amp really rocks.