I wonder if this is indeed the 330nf capacitor added by Mr. Kuehne to solve that DC drift issue.
(At least it would be part of his solution.)
You can see another black cap under the small blue one. That black cap seems to be a replacement for the one Sansui used to bridge diode 07 and uPA68hl.
Mr Kuehne, is that small blue cap part of your solution? If so, then the duo FET you refer to is the uPA68?
Just picking up on this, there were 2 Black Flags on the print side of my Flat Amp board, the small blue one (in the pic) looks like a replacement for one, but also in the pic I see what looks like a Black Flag just below it?