Pattern of 1875, about two years after the recommendation of the Schofield which the Ordnance Dept called the Model 1873. When the Schofield was approved in September, a Pattern 1874 half flap came out for it.
The P1875 was half flapped, but had a single stud hole and a wider body for the Schofield, and a funky flap with a tab extension sewn and riveted on. P1875's were made into June 1878 when the P1878 came out.
The P1878 was bumped by the P1881 Type I with the two hole system. The Type II came out in 1883with a wider loop to accommodate the wider 'Cavalry" or Whittlemore pattern belt.
A third version came out in 1885, called by collectors/historians the "Model 1885" with a wider yet loop to fit over the empty woven belt.
Yeah, empty belt. So they did a P1881 Type Iv with a wider yet loop.
There were a bunch of experimentals tried between 1880 and 1895, like the Fechet, Foryth, Ropes, and Gaston.