Quoted from Section D Question 24-28 ATF Form 4473, revised October 2016:

Firearms manufactured after 1968 by Federal firearms licenses should all be marked with a serial number. Should you acquire a firearm that is legally not marked with a serial number (i.e. pre-1968): you may answer question 26 with"NSN" ( No Serial Number), "N/A" or "None".

Current import rules by ATF would require the markings of the importer, calibre, model, country of origin, manufacturer and serial number to be engraved in a conspicuous way with a minimum size of 1/16" height and 0.003"depth.
Since I do not know if this was a commercially manufactured gun and no importer marks indicate an import by a licensed importer post 1968, I would answer Question 26 in Section D, the one the FFL fills out, as "NSN".

I am more concerned with the fact, that I could only guess that DWM made the frame but that is a guess, nothing more.