The tartan is definitely Royal Stewart.

The only plaid that is obvious is that of the drummer in the RCA-3 picture. It is not a Fly Plaid (a relatively modern invention) but a Drummer's or Half Plaid.

I had never heard of the RCA Pipe Band and will have to update my paper on the RCA tartan. I can't find anything about the Edinburgh Post Office Pipe Band or what they wore. Given the source and small numbers involved in the RCA PB I'd have thought it is possible that the majority of their uniform was borrowed from the EPO PB.