We saw a bunch of Model 10 .38 Special revolvers in Vietnam. Many were issued to US Army aviation crew members, which were mostly 4" square butt, however I saw some round butts also as well as a few Model 12 (alloy frame). We saw a few issued to Army Infantry units for use by "tunnel rats" in clearing underground tunnel and bunker facilities, a use that favored reduced report, muzzle flash, recoil, etc.
In addition to aviator use there were 2" versions issued to CID (Criminal Investigation Division) and some military intelligence personnel, in Vietnam as well as US and European assignments.
Probably other units and uses, perhaps other military branches (Navy NCIS, Department of Defense investigators, Defense Department depots and maintenance facility security personnel, etc). I recall seeing the Field Manuals for Revolver, Caliber .38 which detailed multiple versions of the S&W and Colt revolvers issued during the 1960s and 1970s.
That is one I would get a letter on and hang onto. The letter is likely to tell how many pieces were included in the shipment and to whom that shipment was delivered, most likely a military depot activity but possibly a specific unit or command.