In the early stages of trapdoor production a lot of parts for CW era Springfield's were used, especially on the type 1 long arm trapdoors. You see a lot of early versions with 1863-65 dated lockplates.
Well it appears I have answered my own question. I found where I had read that they were the same. In my Joe Poyer book on the Trapdoor Springfields I read that many of them were built using complete trigger guard assemblies taken from surplus 1863 rifles. The only difference is that they blued them on the 1873 models.