Chris, I would say that you are correct that it is the 500/450 No.1 Express.
The 500/450s do tend to have big throats so one thing to do is to go ahead and load up a round with a .458" cast bullet, fire it, and get a measurement of the fired case.
You should be able to use a soft lead bullet which will allow you to seat in the case with not a lot of pressure. It will be bigger than bore, and probably bigger than groove but if you are going with a NfB load, this seems to be the way to go.
Using black powder will allow you to use a smaller cast bullet which will bump up in the throat and then sqeeze down passing through the bore.
I went through the same thing with a couple of my 450s and now that I am paper patching, I can wrap and size the bullets to whatever diameter the chamber requires. My No.1 Express wants a bigger bullet than my No.2 Musket but honestly, they both shoot pretty well with a .458" bullet too! You may be able to use off the shelf .458" bullets after all. Give it a try.