Has a 52 bore so that means it is a .450
Probably a 500/450 of some sort (No.2 Musket would be my guess or a 500/450 No.1 carbine) of course could be a 577/450 but I’m thinking they got the 500 idea from the breech measurement.

If I was in buy mode, this would be hard to pass on.
That’s pretty reasonable for a rifle that would be a ball to shoot and easy to load for!

Maybe even a better deal
https://www.joesalter.com/category/products/Antique-Westley-Richards-Carbine