My two late-model 261 engines(full-flow filter engines) have the two bosses at G, but only one has the bosses drilled and tapped. One of the blocks does not have either the A or C holes. It does not appear that anything had ever been attached to the G holes. My 1954 261 block does not have the G bosses at all. The rest of the holes on my engines are as ApacheFiend and Racecarl described.

Hoyt