Hmm...I would have to go with the other opinions posted here that this is some sort of one-time customization type of thing; someone had an old officer's cap badge laying around, and mounted it on the sporran.

Most of the US military pipe bands began after WWII; there was a Marine pipe band that was organized in Londonderry, NI during the war (pipers from the Royal Ulster Constabulary pipe band assisted them), but ironically, they did not wear Highland kit, and anyway, wrong badge for the USMC.

Now, there were a couple of American Legion Pipe Bands in the 1920's, but without any proper documentation, the most we can do is speculate.

Bottom line: without documentation, I wouldn't read too much into it.

Regards,

Todd