Any clue what this flag/decoration is?

Looking at the pin, I doubt it was 1883. I would say a guidon flag? But I really don't know much about them.

Here is a Coast Guard burgee. https://images.app.goo.gl/thx2NbCVh7T8F8A76

Reminds me of a burgee used by sailing clubs.

Looks like an Army Cav pendant, and there in may be the issue. Some units had their own unique signature items (some Army Cav units went so far as boots, stetson's, neckerchiefs and such). Have you tried Google Lens?

Could it be that dad swapped insignia with some army guy as a souvenir ?

From my research, which is admittedly limited since I'm in Washington State, that appears to be an Army Cavalry tie clip from the 12th Cavalry Regiment out of Fort Hood. The unit was established in 1901 so that clip would be from 1983.

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.