Since the Marines haven't used Revolvers since the Moro Insurrection, and there is a receipt dated in the late 50's, what you probably have is what he bought at a hardware store someplace.
Willie is absolutely right, and I'll go out on a limb and give you some more information.
A very large number of WWII-era Smith & Wesson 6-shot .38-caliber revolvers were send to the
British military during the war under the Lend-Lease Act, then subsequently reimported and sold on the American commercial market in the 1950s and early 1960s when the UK converted to 9mm semi-automatic pistols. These revolvers were marked UNITED STATES PROPERTY on the topstrap, but this is essentially a legal sham required by the Lend-Lease Act, under the pretense that we were "lending" the revolvers to the British when the US was officially neutral; however, other than a handful of revolvers that somehow rerouted to American domestic security forces, these guns were
never used by the American military.
The former British guns are VERY common, as they were sold by a large variety of retailers- commonly hardware stores and gas stations- for very low prices. Hallmarks are a *.38 S&W CTG.* barrel marking, numerous small crown proof marks, and often a large and ugly 2-1/2 TONS proof at the barrel/frame interface. Most of these guns originally had 5" barrels, but these were often shortened for commercial sale. All of these guns were originally chambered in .38 Smith & Wesson, a shorter, lower-powered and slightly larger-diameter round that the more common .38 Special cartridge; however, many of these guns were converted to .38 Special for commercial sale.
American forces used a number of very similar S&W and Colt revolvers with 4" barrels chambered in .38 Special. These were most often issued to aircrew, military police, and various rear-echelon troops. A few were misleadingly marked U.S.M.C., but this was an abbreviation for the
U.S. Maritime Commission; they were NOT used by the Marines. Many of these guns have also been sold as surplus; there are fewer of them in circulation than the former British guns, as fewer were built and the US military reportedly still has many stashed in warehouses, but there are still enough around that they are encountered with some frequency.
US forces also used a number of large-frame Colt and S&W revolvers during the WWI era. These were originally chambered in .45 ACP (aka .45 Auto) and marked "D.A. .45", but some have been converted to other cartridges. These were marked U.S. ARMY MODEL 1917 on the butt.