The British proof charts have showed 16ga @ .662" since way back about the mid 1800's when they worked out the tables. 17ga is .649" so a .650" bore would have been proofed as a 17 in England. Of course here in the US gunmakers were at liberty to make a bore size whatever they felt like without the necessity of it being marked for size. Early L C Smith specs do show the 16ga as having a .650" bore, but all other sizes are standard per the British charts.
My H grade Lefever bearing a 1907 patent date currently has bores of .669" in both bbls. I cannot of course attest they are original, but both bbls have 4+ inches of choke with .035" constriction (.634"). I really don't think it started life with bores .019" smaller than current. It is a rather high condition gun, not showing abuse or even very heavy use.
