I'm not sure if there are still records on how many of each watch model was made but maybe one of our Hamilton enthusiasts here might know. I can say that there were 213,657 of that model 987 movement made beginning in 1926 and running until 1930 per the NAWCC records.
Long ago it was believed that a few dozen of the early solid gold "Geometrics" were made in 2-tone solid 14k gold. These included the Square, Square B, Oval, Barrel, Tonneau, Cushion, etc. Over the years this evolved after some trial end error. The new belief is that ALL of these watches with what appears to be very lightly yellow or green gold centers are actually merely 14k white gold cases with centers that tarnished slightly different than the backs or centers. No one knows why this happens, but the thought is that perhaps the casemakers (generally Fahy's) might have used a slightly different mix of metals in the centers, perhaps to strengthen the lugs. To see if your watch is actually white gold, just remove the movement and dial take a little simichrome polich and gently rub the case with a polishing cloth. The center should turn as white as the back and bezel.
All of that said, one of the leading Hamilton experts told me once maybe 10 years ago that he saw mention of a very few genuine 2-tone Square and/or Square B cases in the early stash of Hamilton documents, much of which was complied by Mr. Halligan, later rescued in a Lancaster garage by Dr. Ravel, and much later donated to the NAWCC museum by Mr. Rene Rondeau. Therefore, if your Square's center remains yellow/green after a light polish, then perhaps it's genuine? I own maybe 8-10 of what would of at one time been called a Hamilton 2-tone, but after a light polish I think 2 of them (a Square and Square B) might be genuine while the others were merely just tarnished and returned to all white after a light rub. The jury is still out, I guess.