This particular Maynard had three dates impressed into the action 1851 and 1859 patent dates and then the 1873 patent date when it was back at Mass Arms for updated modifications. It was an early Civil War era arm of the percussion cap style originally and later converted to the firing pin style for Maynard head primed style cases. The Serial No. is 20XX, so that would indicate that it came home from the battlefield with some soldier or it went to the government auction as war surplus.
The barrel was most likely a replacement when it became a 1873 conversion. I believe what this is about is a '73 sporterized owner visiting his friendly gunsmith with a complain that he could no longer find Maynard "pan head" cases for his beloved Maynard, and the gunsmith indicating "no problem", I can solve that.
The process being, the barrel was shortened at the breech end and a spacer piece was made in the correct thickness in order to set the head space. There were no pins nor screws attaching the new spacer, so it was silver soldered in place.
Next a chamber reamer was used to cut the new chamber and the rim recess, thereby eliminating the need for the old style Maynard dish pan head cases which were no longer available.
Of course, the gunsmith then realized what he had done and being a bright young up-start, said to himself, "Self, what's going to keep the breech closed?" So wallah, the finger lever catch mechanism comes to life.
That's my story and I'm sticking to it.
C.M.M.