Oregon Bill,
Herbert Houze's "The Winchester Model 1876 Centennial Rifle" covers the earlier developments of these big bore lever rifles. You are correct, the design that became the Model 1876 pre-dated the Model 1873. The Model 1873 was introduced commercially first. The design that eventually became the Model 1876 was based on Model 1866 "Swiss" rifles, as well as the Model 1868. Not many of these were made and were not released commercially. There is also an earlier cartridge called by collectors today the .46 OFW for Oliver F. Winchester. It predates the .45-75.
Also, there was a Model 1878 Winchester in .45-70 which was submitted to ordnance trials. It had a longer receiver but looks just like an 1876 action. It was totally capable of handling the power of the .45-70 cartridge. That's another old myth, that the 1876 action was "not strong enough" to handle the .45-70 cartridge. What hog wash!