It may help, though I'm not sure if any of the post war conversions (to the Allin Trapdoor system) had that date stamped on the M1863 lockplates. I'm certainly no expert on the Trapdoor rifles, but I'm sure somebody on here can say whether or not Springfield used newly stamped plates. Closeups of both sides of the breech area and a measurement between the band spring mortises would probably help. I couldn't read all the numbers on your tape measure. Also, as I mentioned earlier, even when I zoomed on the picture I couldn't make out whether or not there was a boss on the trigger guard where the sling swivel would have been mounted. Is there one at the rear of the trigger bow? I've seen a few where someone removed the bow from the plate and reversed it when they put it back together. This is an interesting piece. It's obviously been changed from whatever it was originally and it's always fun to try to figure out what is what.
I'm looking forward to the new pics and other info.