The gun likely has 2 9/16" chambers but a gunsmith could confirm.
Choke constriction was not indicated on Smith guns so the 'full' mark is likely after-market, but .625" would be full choke for a 16g.
The "D" is the mark of Jean-Baptiste Delcour-Dupont/ Canons Delcour S.A. of Nessonvaux, Belgium; the maker of the "rough forged tubes".
The mark has been found on Smith (Crown Steel and post-WWI Armor Steel), Fox, Meriden/Sears, Baker ("Homotensile Steel"), Ithaca NID ("Best Fluid Steel") and Lefever Nitro Special & A-grades.
Please don't use the gun with any load until it has been evaluated by a double gun specialist smith.
The is lot of information in the FAQ section on the Home Page, including load selection. The gun was not designed for the fps of many modern loads which produce substantial recoil.