Wondering just what I have 45-70 or?

Hi and thanks for letting me in.
I have a cartridge that no one can really ID.(so far)
There are many guesses bit no solid answer, so here goes:
head stamp - R-P (arc on top) 45-70 GOVT (arc on bottom)
Looks like a 45-70 BUT … see the dimensions:
case OAL=2.392 (standard case 2.105)
cartridge OAL 2.755 (standard OAL 2.550)
bullet dia 0.443-0.444 (or so) (standard 0.458)
case OD dia at neck 0.474 (standard 0.480)
case OD dia at base 0.5 (standard 0.505)

and it appears to be necked down (on an angle) at about 1.95 from 0.482 to 0.474 (on a taper)
large primer
have not pulled this yet,
This is NOT a standard 45-70 as stated on the case base, I HAVE one of those (1884 Springfield Trapdoor) and there is NO resemblance, after one look . .
Any help is appreciated, I have pictures and dimensions if one needs
THANKS again.

Hi John, sounds interesting. Please, can you post pictures?

44-90 Remington necked from stretched 45-70 brass?
COTW puts it close.