Pat, I dated my site to 1800 to 1830. There were some items older and some newer but the date given was the average.
I'm the first to challenge people's romantic notions about their finds, having an analytical mind and adding other variables but this time I'm going to support the "Chief's presentation gun" theory.
I found my piece at a known Native meeting place. I did find other musket rifle parts but the only other escutcheon piece I found there was this.....
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An article about this silver escutcheon appears in the Museum of the Fur Trade quarterly stating that it belonged to a chief's presentation gun.
My piece has been altered as they shaved the edges off for some odd reason.
More evidence on this notion is given your location and mine, as there were no Rev. war/of 1812 battles or forts. Nothing but Indian territory.
I could take a picture of the article that talks about my silver piece if you like.
Searching all my books, the only other "GR" plate I could find was in "Colonial Frontier Guns" by T.M. Hamilton. The picture won't reproduce here, but the accompanying text reads...."Another British military escutcheon. It has the King's mark, a crown over "GR", engraved on it's face. From the Brown site, 23Ve3, Nevada, Missouri.
I would love to read that article you mentioned about guns with that escutcheon being presented to chiefs.
I'll let you know if I find out anything else.
Dave.