New Friends,

About 3 weeks ago, I purchased a .50-70 caliber NY-marked rifled musket that looks like a typical NY Militia rifle EXCEPT that:

1. It shows NO sign whatever of ever having had any finish applied & is "in the white" all over,
2. The barrel is stamped "USN" adjacent to the receiver on the left side & has a capital "P" in the same area on the right side of the barrel.
3. The left side of the receiver has a block "B" stamped into the metal.
4. The right side of the receiver is stamped "P" over "F.C.W", just above the trigger guard.
5. The top of the buttplate is stamped "US" in front of the screw & "S" in the center to the rear of the screw.
6. The rear portion of the buttstock is stamped "NY" over "74th REGT" over "USN".
7. The bottom of the stock is stamped "19" & there is a stamp near the receiver tang that is "P" over "NY".
8. The hammer is the typical "tall" NYS Militia style & the block's lever is also typical of NY rifles with the "shield on the horizontal lever.
9. The barrel is about 34 inches long & (after looking carefully with a 20X magnifier) seems to have never had a bayonet lug.
and
10. The 3-band rifled-musket seems to be unmodified (except that it has NO swivels)  & is "sound" in every respect, with an excellent bore.

Over the last 3 weeks, I've consulted every "usual" Internet search engine & George Layman's book, MILITARY ROLLING BLOCK RIFLES & have found NO identical rifled-musket in any source. 
(I've of course found any number of "somewhat similar" Number 1 military RB rifles.)

Can anyone tell me,  from my written description, EXACTLY what I've bought?? 
(I'm quite curious about what exactly that the RB is.) 

ADDENDA: The young man's family, from whom I purchased the RB, knows NOTHING about the rifle except that the PO's great-uncle owned & hunted deer with the rifle before WWII & that after his death in 1954 that the rifle was stored in the attic & left there until recently, when the house was sold.

THANKS for reading this.

yours sincerely, tex