Good point!
I suppose this gun could have originally been a 303 but the "express" inscription on the side makes me think it was the 450.
The reason I sort of hope that it was a 303 is that the extractor is less likely to be buggered up in conversion to 45-70 and would be an easy conversion to one of the Westley 500/450s.
If the smith went from a 500/450 to 45-70 there is no telling how he did that judging by the barrel job!!
Fingers crossed!