It looks to me that the .410 started out as a 2" to be shot in 45/70s
COTW states that the .410 shotgun was developed from 44XL Shot, a 44-40 case with a paper or wood shot capsule. OAL was just over 2".
It is certainly possible to fire 410 shotshells in a 45-70 with no ill effects.
I do believe there were 45-70 shot cartridges loaded by the US mililtary. Someone who is an avid cartridge collector should chime in any time.
I must be getting old.
2 years ago at the Puyallup gun show, my brother bought a 44 shotgun for $110, and borrowed the money from me.
I figured out how to load it, and in the process learned some of what is in your post.
My brain somehow mangled the information.
What does it all mean?
I was wrong about the 45/70 fathering the .410.