That's an 1873 patent Westley.
I have my eye on a very similar gun - unless you just bought it...
Do a chamber cast for sure. Wouldn't be surprised if it ends up being a 500/450 of some sort (no.1, no.2 musket etc). I have an 1869 in No.1 express and .458" bullets work very well. .458's work great in my No.2 musket farquharson too. The fellow with the 1874 I am looking at slugged his and it is also .458" chambered in the No. 2 musket round.
Very nice gun. If the bore is as good as you say, it should shoot like a dream.