I can't think of anything else it would be. That's pretty typical of most GM cars.

If there are no other hash marks, I'd bet A is where it should be set when dizzy vac advance is disconnected and idle speed is set. That's probably 5* BTDC or whatever the engine is spec'd for.

1950 Chevrolet 3100 (Ol' Roy)

1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)