1960's Contract M-1943 Entrenching Tool cover. This version was made for the USMC for use with the M41 pack system, so it was a USMC only version. While other services were using the M-1956 version with the more modern ALICE type clips the USMC was still using their M-1941 packs that required the WWII style belt hook.
In very good + condition, most have some very minor patina from storage. Picture shown is an example, I have multiple from the original case so the one shown may not be the exact one you get, but it will be the same model and similar condition,