Now THAT'S interesting. 2 1/8" appears by the stamp to be the case length. .45/70, of course is 2.1" but, should chamber.
Just wondering if the .45/70 case is too small in the base.

You should be able to get the tines of a caliper into the breech well enough to measure it. A .45/70 should have a .500" to .508" chamber base diameter, whereas the .450 Ex. case is .544", but could be as small as .533".