[JMPRO]

I just heard back from a guy I know. Robb Lawrence, celebrated author of at least a dozen books, 4 of which are about Gibson guitars and Les Paul. Robb was a long time friend of Les Paul and Leo Fender and can tell you what kind of toilet paper they used at the Gibson factory in 1949, well maybe not that much detail but he has done so much research he probably knows more than anyone about Gibson.
Robb response is that the "SG" stands for Spanish guitar not solid guitar. Spanish guitar was a very common name years ago to distinguish between what was called back then the Hawaiian steel guitar and the upright fretted guitar. When I was a kid people did not use the name "Solid body" to refer to any guitar. It was simply a Spanish guitar, acoustic or not. When I worked at Fender in 1960 nobody said "Hey, hand me that solid body".
Jerry