DCM is Director of Civilian Marksmanship. AFAIK, the office still exists but is no longer the Army agent for sale of surplus firearms to civilians. At one time, in order to get a surplus gun from the Army, you sent an application through the NRA. They certified that you were a member, which was a requirement, then forwarded the application to DCM. If the type of gun you applied for was available, they cashed the check and sent a request to the appropriate Army depot to ship the gun.
DCM itself never handled or even saw the thousands of guns that were sold that way. They had a small office on the first floor of the Pentagon, with a half dozen staffers and rows of filing cabinets.
Now on the M14's, since so much has been written that is wrong. The Army and ATTD (predecessor to BATFE) had worked out a deal. The Army, as the official manufacturer of the rifle (the contractors merely worked for the Army) would take M14's, weld up the selector, rename them "M14M" so they were no longer machineguns, and sell them through DCM. (That could still be done today if the political atmosphere changed.) At the same time, they were preparing to sell M1903A4 rifles with scope sights.
But what happened was Nov. 22, 1963. After that, the Secretary of the Army (a puppet of Lyndon Johnson) directed that M14 sales be halted (it had never actually begun) and that M1903A4 rifles be stripped of the scopes, the scopes broken up, and the lenses given to schools for "educational purposes". (No, I am not kidding; you can look it up in the old American Rifleman magazines.) A few M1903A4' with scopes had been sold before the ban, and they are scarce today. (Most on the market today were assembled using commercial scopes.) That is why there are plenty of M1903A4 rifles around but very few original scopes.
No one cared that there would be no "educational" value in old lenses; the whole thing was political, with Johnson working to placate the left wing and the Kennedy fans who believed (and some still do) that he had had JFK killed.
Jim