I don't know anything about the different barrel diameters or how to mount the 1903 sight base but I think the two types of front sight bases offered by Sarco are for specific to the 1903 Springfield and the 1903A3 Springifeld.  The 1903 sight base has a dovetail cut into it for the sight key, a separate piece which actually holds the sight blade.  On the 03A3, the sight blade is pinned directly into the sight base, not a separate dovetailed sight key.