IMHO, it has to do with age. The open "a" is the older.
On the large Mobilgas signs the open "a" is on the pre-WWII signs. Pump plates in the shield form wasn't available before WWII, and some early west coast and MC-AX plates can be seen with open "a", but not on the I.R. signs.
In magazine advertizing after WWII, the open "a" is used only during the 40s, and even then, mixed in with some cloesd a's.
Maybe the age can be verified by serial number on the pump, but of course, the top may have been exchanged over time?
br,
Claes