1) The 1969 amps used .0022s
Right -- and they had metal panels. . .
To say "plexi = 0.022" and "metal panel = "0.0022" as regards the bright channel of lead (split) circuits seems "the most correct."
2) the filtering also went up before 1971, I think it was higher when the alu era started in 1969
Yes, hence "by
5) Didn't some 1973 amps use a .68uf on the 5k pot? Wasn't this what george lynch prefered? Or have I got it wrong?
I actually had a 1971 in that was wired that way, though I don't know if the wiring was original or altered later, as the amp was a mess
6) the 5000pf was ordinary by 1969
Other values are still used past 1969. . .Lets face it there is no cut and dried explanation that will satisfy each amp that Marshall produced (unless you use an explanation that includes alcohol). The best a person can do is watch their use of qualifiers and determiners (count language versus noncount language), and make sure you don't say "all," but "many," and instead of "none," say "few," etc etc.
I think if a person wanted to change their 50 watt from a "1968 spec" as seen in the 50 watt kit instructions to a "1971 spec" that our pointers, as laid out, would get them 95% of the way there. Surely we could always find strange exceptions. Or we could find amps that have been rewired, so that we may never know how they left the factory.
As an aside I found the .68 on the presence pot to be simply hideous in tone!