Steve,
As far as I am aware, the shields are from the same time period. If I had to take an educated guess, I would agree it was probably just a difference in manufacturers. Both are the larger "white border" pump plate shields used in Socony-Vacuum's Magnolia marketing territory. The open a is called the serif A version, the other is known as a closed A to Mobil collectors. Although it doesn't apply to these signs, the serif A would usually be older. It has always perplexed me that Socony-Vacuum used the earlier serif A on the Mobilgas globes from 1934 to 1962, but the use of the serif A on the large pole shields ended in 1946 from the research I've done over the years. They used the closed A from there on out. Incidentally, the wide blue border pump plates which are the most common Mobilgas plates, used the closed A during all years of production from 1946 to 1954, and into the thin blue border versions from 1954 to 1962. I'm sure a booklet could be written on Mobil pump plates alone.....there are many variations to collect!
Darin