Let me make a disclaimer before I continue with my thoughts: I am in no way, shape, or form, attempting to begin an argument, nor am I "Twisting the Tiger's Tail"!
I have a different opinion on the M-1855 Rifle Musket and the 1857 production at Harper's Ferry. It is correct that there are no production numbers shown for the M-1855 RM for the "Calendar Year" of 1857 for Harper's Ferry. (Miller, Vol. 3, pg. 254). What is shown are the production numbers for the "Fiscal Year" of 1858. The "Fiscal Year" of 1858 ran from July 1, 1857 to June 30, 1858. I am of the firm belief that Harper's Ferry did in fact produce a yet to be known number of Rifle Muskets dated 1857 during the first six months of the "Fiscal Year of 1858", which were in fact the last six months of 1857. I believe it is a simple mis-interpretation of the records by overlooking the fact that the data is for the "Fiscal Year", and not the "Calendar Year".
My personal belief for this is supported by the number of Harper's Ferry M-1855 Rifle Muskets I have seen over the past 50+ years which are dated 1857. I have also seen a H.F. 1857 dated example in a private collection that is in near unfired condition, and is fully Harper's Ferry from one end to the other. I have also seen far too many loose H.F. 1857 dated locks around for all of them to be only from the Harper's Ferry Rifle.
Again, just my personal thoughts on this, but think about the difference in the records between "Calendar Years" and "Fiscal Years"!
J.