Oh, and to answer some of your original questions,
yes it is legitimate. It could have been military.
Branch issued to could be any one - the post-war knife was available for issue to any government agency ordering stuff from the GSA catalog. It could have been (most likely) the Marines, the Army, even the Air Force. Heck, since it's in the GSA catalog (still to this day) it could have been ordered by the US Forestry Service, the IRS, the Border Patrol, the Coast Guard, or even the National Park Service.