These plaid things would make sense if there existed in the military such a thing as a "piper's plaid" which was worn only by pipers and a "drummer's plaid" which was worn only by drummers...
but there wasn't.
Rather, there were "full plaids" or "long plaids" which were worn by officers in certain forms of dress (especially with trews or tartan riding breeches), certain senior NCOs, and, yes, pipers.
And there were "belted plaids" (so called because they were secured around the waist with a narrow cloth belt) which were worn by officers in certain forms of dress, all NCOs and privates in Full Dress, and, yes, by drummers (due to their being soldiers, not because they were drummers).
Indeed it's rare to see a military piper wearing a belted plaid but here it is! a piper of The Cameron Highlanders. Note that he's wearing the simpler form of jacket that the Highland regiments wore on active service in the latter half of the 19th century

And here's a piper of The Cameron Highlanders in the usual Full Dress, for comparison
Proud Mountaineer from the Highlands of West Virginia; son of the Revolution and Civil War; first Europeans on the Guyandotte
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