Last but not least they didn't let me take pictures...
That, in itself would be a huge red flag to me. No antique dealer knows everything, especially where arms are concerned. I once drove to southern RI to look at a "Brown Bess" in a local gun shop. The "gun expert" was sure that's what it was...and couldn't have been more wrong. It was actually a M1777 French musket but since the price was $175 I bought it without telling them anything. This was about 25 years ago and, even then, it was an insanely low price. My rule of thumb is that if a dealer places a price on something and he's way off that's his problem.
No Russian muskets were used in the Revolution. I doubt anyone has ever made that claim. Ironically enough, the British government approached the Russian government to "rent" Russian regiments during the Revolution. Catherine II turned them down and they then went to the various German principalities who supplied what we call the Hessians (though not all of them came from Hess). But for that there would have been a "Russian intervention" in the Revolution.