External serial numbers with letter suffix, no date, .30; I think it is one of the many WWI leftovers rebarreled to .30 under Versailles restrictions.
I see what you mean, but the gun has commercial SN placement. I was not aware that after WWI, finished lugers had all markings removed and then were numbered in a commercial style?
The OP's luger lacks military acceptance proofs on the right of frame. The gun has Crown/N proofing as well, which is the commercial nitro proof. The gun, while having readable SN numbers on parts, still actually has what is considered "hidden" SN placement which is a trademark of commercial lugers. When ascertaining hidden vs exposed (military) look at the side plate and the take down lever. When the numbers are on bottom aka this gun, those are called "hidden". When they are outside meaning visible from a left side view of the gun, the numbers are then "exposed".
Here is "exposed" on my Mauser G date (1935)
Here are the "hidden" numbers of the OPs DWM luger