From email:

If you see the pic, just imagine that you can read the white printed
letters in front of you. So
it would read something like
C=CSG
318069-02
8372A
5091 23
!!!!!!!! !!!!!!!!!!!!
the missing pin would be just below the number 5. Hope this gives you a
clue!
From a quick check it looks like what I suspected is correct. It looks like someone tried to do a PAL hack. read this.
HEREGood info on the Agnes in A500.
Maybe they phsyically damaged the Agnes or the socket in the process. Does the A500 have a memory expansion in the belly slot? If not, from my experience, then even with this pin lifted it should still work and should still default to NTSC. My experience is limited to only rev 5.A(iirc).
I'd start off trying another Agnes.
Do you have an A501 in place?