I doubt it was ever headstamped that way.Sharps just called it the 2.1, Remington and Wincheter referred to it as the 45 govt.
There were some variations of the 45-90 winchester that had differing headstamps tho, and the 45-75 would likely have been for a heavier than the 280 gr bullet.I have a 45-85 marlin cartridge, which was their way of saying 45-90 winchester , without doing so...
And don't forget that there was the 45-75 winchester but it was a bottle neck
