I have a Hamilton 910 movement . The 910 is supposed to be a 12s pocket watch but when I tried to use a 12 size case it wouldn't fit . The 910 has a rather large dial on it and the base plate looks larger around than the bridges cover . What should I be looking for in a case for it?

You'll need a Hamilton case.

That would be too easy OK so I guess all 12s cases are not the same . Lesson learned .

One of our Members posted in our " Parts Wanted "
area looking for one, I helped him out and sold the only one that I had. It was a Hamilton Marked case.
You can give that a try to find one. Jack

Brian, 12 size watches made after about 1905 are a "Royal Pain" to re-case. The pell-mell efforts of the watchmakers [watch companies] to make thicker watches look "Thin", thinner watches look paper-thin and/or shaped like pears, penta-hexa-octa-hallucigons, clamshells and/or any other "new" looking aberrations of common sense has left me and many others with a plethora of "12 size" movements, cases, and the growing disappointment that NONE shall ever find another "home" or correct movement.
We often say that 12 size watches are overlooked as valuable collectors, but I now see them more often as a precursor to the cosmically infinite variety of shapes and sizes which finally were fully realized (to this day) by wristwatches.

I have a Hamilton marked gold filled case along with the crown that was designed for the 17/21/23 Hamilton 12 size watches.

I had planned on picking up a movement to put in the case but never did so it sits on my computer stand gathering dust.

Claude, the Hamilton 917,21, and 23 are 10 size watches. Brad