Nice sign Al.
The rectangular signs like your's replaced the 5' rectangular SOCONY pole signs. The switch over to the use of Pegasus on signs started soon after the Socony and Vacuum merger in 1931. The earliest shield pole signs featuring Pegasus are from 1932. Some of them say Mobilgas, some still say SOCONY, but both have the Pegasus logo. The rectangular version like yours I've seen IR32 and IR33 and IMO were only used a short time during the initial changeover.
As Socony-Vacuum set their station image standards, the ones sporting these rectangular signs would have had to switch to a Mobilgas shield eventually. In the line of shields, the research I've done over the years has shown they were made from '32 up through the mid-50's when the "flat shield" logo was adopted. The only years I've not found examples would be during the war years. I've seen an IR41, but none from '42 through '45 inclusive. Obviously the war effort more than likely halted the production of porcelain signs during this time. On a side note, I've seen 2 different versions of the IR46 signs. Some sport the earlier serif A, or open A as your sign exhibits. Some use the new font introduced that year with the closed A.
I hope I've given you some answers to your question.....