[Edward Tinker]

Blane,
The 1920 usually indicates a property mark and not a date marking, but it could very well be a date marking also. Pictures would help very much.

9. Markings: On barrel and chamber is the German "Nitro" commercial proof #28 (Crown N). There is a letter at the base of the barrel just under the serial number that I cannot make out.

Commercial guns did make it into military usage during the Weimar era, and it sounds like you have military serial number placement. If you can provide a scan or pictures, that would be helpful. If all matching Lugers usually are worth $600 - $1100 (and up for specific models), but of course depends on condition, condition, condition, so with such a low bluing, it would be at the lower end of the value.

If you would like to send pictures to me, I would gladly post them here for you,

Ed