IMO, although your GPR wouldn't have had much in the way of inlays if it was being used by the Trappers, its style is in the early era when the Ohio rifle was being developed for use in more civilized areas
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The Ohio rifles were typically smaller bore half stock guns which did use inlays so having them on your gun would not be out of place.
I don't have any photos of these Ohio guns so I'm totally guessing, but I would think stars, Eagles, acorns, hearts, ovals and a number of similar inlays would have been used.
By this time, German Silver had been invented so you might want to keep that in mind.
Although Sterling Silver inlays are nice (and available in inlays from Muzzleloader Builders Supply) the silver does tarnish rapidly when it's around black powder smoke.
Bottom Line? Because your gun is a Flinter, it represents the late Flintlock era so inlays common to the earlier guns would not be out of place. It also represents the Half Stocks that replaced the fullstocks so simple inlays would be appropriate.
In other words, use whatever makes you happy. After all, it's you that you need to please. ::