Hi Chris,
Most lightweight motorcycles in 1911 would have had 13g spokes. Even medium weight bikes - 3 1/2 h.p. Triumphs and the like - would typically use 12g. I doubt that any 1911 motorcycle would use 10g in the front wheel; perhaps a v-twin sidecar hauler. Your measurement of 2.56mm (about 101 thou) is a bit under the usual size for 12g (0.104") but bigger than 13g (0.092").
Some spokes were "butted" - thicker at the hub end for an inch or so. If the hub is original it may be that the original spokes were butted, say 12/13g.
Hopefully someone can come up with a number from period literature.
Edit: Forgot to say that nipples come in different diameters for any given spoke gauge, usually in 0.025" steps. If you want to use 13g spokes, you need to specify the nipples as "13g, 0.225"".
Leon