USN Victory shoulder holsters were marked on the rear of the holster in the upper left hand corner along with the date. These holsters were reported to cost the Government $1.35 per unit in 1943. It would not have been issued with the ammunition carrier. There were generally made by hand by the "Riggers" on board the ships and constructed from "sea bags" and elastic. The number of rounds of ammunition generally varies from example to example since they were made by hand.
Belt style holsters with the full flap were not marked. There has been one example reported to have a small US stamped on the rear piece of the leather that attaches the metal fastener to the holster, but I have never seen a photo confirming it. It was reported to be so small a magnifying glass was needed to see it.
There have been many WWII holsters that were marked up in some configuration by the soldiers that carried them such as unit numbers, divisions, initials, etc but they are just that, personal and not as issued.
There are half flap versions of the belt holster (no photos available) that was produced in WWII that are marked Boyt 43 and have a oval with USN embossed on the front of the holster, not the flap. Similar half flap holsters were also made with slight variations by Rock Island Arsenal and several other contractors from 1942 thru 1970's and were marked with an oval containing US on the front of the holster, not the flap.
If you are looking for a half flap holster from WWII, you will want to find one that is dated or it may be one of the models made later on into the 1970's.
If you desire a WWII full flap version, it will be unmarked.