First make sure you're looking at the "VIDEO" board, and not the "CPU" board.
Horizontally across the bottom of the board, you'll see the numbers 1-7.
Vertically from top to bottom, you'll see the letters A-S.
Go down to "P" and over to "3."
The ROM chip you're looking for should have a sticker on it that says "3Pt". It will be next to capacitor 50 (C50). It's the one furthest down closest to the ribbon cables.
EDIT: Oh, OK, cool.