It looks like a military kilt, with the typical military two-prong black buckle.

We can't tell from the photos whether or not the kilt has the usual buckle on the wearer's left, I would imagine it does.

And it might or might not have originally had two buckles on the wearer's right.

That's the thing with ex-Army items, often they end up in civilian hands and are modified.

A musical instrument in a museum tells us little about how it originally was tuned, sounded, and played; to know these things we have to hear the instrument being played by a good musician who specialises in that instrument.

In like manner an item of military clothing in a museum tells us little about how it was originally constituted and worn; to know those things we have to see that item being worn by a member of that particular unit in the same time-period the clothing item dates to.