U5 RAM?

[der Mouse <mouse%Rodents-Montreal.ORG@localhost>]

I've got a U5 that I'd like to get going.  But when I turn it on, it
loops reporting "RED State Exception".  On inspection, it turns out to
have no RAM in it.

Superficially, it looks as though it takes 168-pin PC RAM.  But the
pattern of notch positions seems to be unusual; I've gone through my
whole collection of PC RAM, and, while I've found plenty of sticks that
match either notch position, I've found none that match both.  Am I
just unlucky, or is U5-compatible RAM rare?

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