Well it could be but more commonly I'd expect A-board and I have no solutions for you either. I had a 54S10 recently 1 blink I was mucking around with. Total basket case. badly corroded, both A board and P board were duff and it was 1 blink.
Now I don't keep notes but I had an A board from a 50 and still 1 blinked, jerry rigged the 50 P board for a quick test and it worked but then panel was screwed with vertical lines.
Put the original 54 A board back in, 1 blink again. That was a surprise.
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