newbie trying to date a 1917 DWM

[Edward Tinker]

It is likely 9mm, but unsure do a cast of the chamber, or take it to a gunsmith.
Its not a 7.65mm luger, as that whole tip would go inside.

The suffix under the serial number was placed there every 10,000 lugers (each year), so assume 140,000 were made, then 1-9999, then 1a etc - its explained in the FAQ