I want to load some 45-70 to duplicate the original Govt 45-70-500 load in my trapdoor. Is the Lyman 457125 500grn round nose bullet close to the same as what was used for this load?

This is a quote directly from Lyman talking about the 457125:

"This is the original standard 500 grain 45-70 Government bullet. This has been in the mould line for nearly 100 years."

So I'd say that's the one your after. ;)