It's essentially the same information I gave you on your earlier query on the 'BROW' forum...
You use the Model 1879 rear sight because that's the sight supplied by Springfield Armory when they built those rifles.
It's a more usable sight than the Model 1873.
S&S Firearms - and possibly Dixie - should have a ready supply of them still.
All a machinist will need to know is where the first hole measures to - you can get that by checking an original, or when I get home, I'll measure mine.
All Trapdoor rear sights have the same-length distance for rear sight installation - when a new sight came up for issue, all that was needed was to unscrew the screws and drop the new on in place.
No record of any of the Buffington rear sights is noted - they came about for the Model 1884 rifles.
Vaya,
Scouts Out!