Any clue what this flag/decoration is?

It looks to me like a Civil-War-Era-inspired regimental guidon, mounted on a tie clasp, and I'm guessing that it's probably from a unit in the Class of 1983 at West Point.

I went to the Medals of America website and there was nothing like it there. My guess is the 83 is the year it was presented. I think the 12 represents a Squadron, Unit, Battalion or Command of the United States.

It looks like tie tack memorabilia. The "Flag" is a representation of a 12th Cavalry Regiment guidon.

The black field with the number '83 is not a standard feature of a Cav guidon. I suspect that this is a reference to the year 1983. The person who had this memorabilia was either an Alumni of the 12th Cav in 1983; attended a 12th CAV function in 1983; or if the owner earned their spurs in a 1983 spur ride.

If I had to bet a steak dinner, I'd guess this is associated with a spur ride. How a Marine who served in the late 1940s and early 1950s had appears a mystery. I would guess it was provided as a gift or reward (similar to a challenge coin).

Possible tribute to the U.S. Embassy Beirut bombing in April 1983, or the October 1983 bombing of the Marine Barracks at the Beirut Airport.

The shape, the colors white and red above (similar to yours,) with the red and white horizontally bisecting and the guideon, are traditional US Army cavalry regiment. Some had black vertical bands near the standard for different reasons. Like today, it wasn't unheard of for servicemen to have been in both services. I suspect this may be something like a specific unit clasp. Such things didn't always use official symbology. The inversion of the US Army Cav colors may have meaning.

Guideon is clearly a Cavalry Troop Guide-On, from 1983 belonging to the U.S. 12th Cavalry. Anything more would have to come out of Army Heraldry in DC. I know for a fact that the Army, in The Pentagon's Library, has EVERY SHOULDER PATCH, MISSION PATCH, BRANCH PATCH AND UNIT PATCH on file there. An Inquiry to the Army's Public Information Office might be a good place to start given that They and the Library are about 4 Ring's from each other, and on the same floor.

12th US Cavalry
The 83 is year served

On 15 June 1983, the 1st Battalion, 12th Cavalry Regiment was relieved from assignment to the 1st Cavalry Division at Fort Hood, Texas. Reorganized in 92

Looks like a 1980's era enameled memento luggage tag. That was a thing then, I remember my dad having a handful of them from various meetings and conventions. I'm guessing that was a keepsake from a 1983 12th CAV reunion of some sorts, but they got the red/white reversed on the order. Hard to tell from the photo, but that strap sure looks like pleather so I'd think 1983 for sure. If the back of that has painted area with lines for writing your name and address, that's for sure what it is.

Enameled memento luggage tags were a thing in the 80's, I remember getting a lot at industry conventions. That looks like a luggage tag and I'd guess it's swag from a 1983 12th CAV reunion of some sorts. Guessing they got the red/white backwards on the order.

Just a thought but maybe take it to a military museum and let them look at it. Everyone thinks the '83 means 1983. The rest is unclear since the colors are wrong, but the museum should shed light on it. Maybe it was taken as a drought from some training exercise. Semper Fi brother!