Lamps are kind of confusing, a common ANSI convention for lamps uses watts/shape/diameter. The diameter is spec'd in eighths of an inch. See these Amazon searches as examples 60A19, 25T10, 7C7 . (Ballasted lamp types precede W/S/D with a letter like F32t8.)
But then the base isn't part of the number. They are now metric "normalized" dimensions. Edison didn't define the E26 base as an Edison 26mm, it was an Edison 1". An E17 was a 5/8" intermediate base, and an E12 was a half inch candelabra base.
The glass on your bulb lacks a neck, I don't think it's a "G" Globe format, I think it's an "S" Sign format. So if 1.25" lamp in watts/shape/size format would create something like a 40S12. I'm guessing it would actually be a 40s11 since the base looks to me like an intermediate base, but it could be a candelabra.