That's a good question, a chicken-or-the-egg thing.

My assumption has always been that DC Dalgliesh created that palette (as I recall introduced in 1949) and that at some point other mills like Lochcarron copied it, dubbing it "weathered".

In the 1980s I worked at a Highland Outfitter, we mainly used Lochcarron, House of Edgar, and Strathmore for our fabric and as best I recall Strathmore also had weathered tartans (or perhaps it was some other mill, I do know we didn't deal with Dalgliesh).

But as you rightly point out this is just conjecture because I don't actually know when those mills introduced "weathered" tartans.

Old catalogues won't help because they don't state who weaves the fabric they use for their kilts.