I dont think ther was an access hole. If you have the holes on the firewall then it had the filter mounted there. If not then it was the tank one.
This is from the G503 site.
The top and bottom of the filter was made from thin (1/8 inch), circular pieces of porous, bronze. These circular bronze pieces were then soldered to the center portion of the filter which was made of sheet metal. The filter was attached to the inner fuel pick up line by flairing the line after the center part of the filter was attached. My guess is that the inner fuel line and fuel filter were assembled as a unit before they were installed in the tank. This must have been done before the sides of the tank were put on.