I would not worry about the lack of a serial number either. This thing was built by the government and released to be distributed by the DCM to end up in the hands of civilians.

Another disquieting thought, maybe the step was not forgotten but another defect was encountered and the step skipped on purpose. Were the proof marks just done on the barrel? Was the barrel from another gun and this receiver never proofed?
If a dangerous defect was detected and the receiver was not completed on purpose then it should not have been heat treated. Since mark is having tests run checking for heat treat would be in order. My self I would try a small file test in an obscure area of the receiver, at a automotive lab they will have hardness testers but they will have to keep in mind the shallow case with a very soft core is normal for Krags. It will be very different than the modern automotive steels made today.
Right now I am inclined to think the DCM/NRA, Bannerman, or Stokes made a custom stock for the carbine and released a safe gun to the public.
But as Charlie Brown said "It is a mystery."