Unmarked M1867 Remington Rolling Block

Attached are pictures of a black powder Remington rolling block I got recently with a yatghan bayonet made by Alex Coppel of Soligen that appears to be stamped with a crown. I believe the bayonet is Danish but am not sure that it is original to the gun.
The rifle has no markings except those on the tang that I've pictured, which tell me it is an M1867 made by Remington. I have taken the fore stock off and there are no proof, acceptance, serial number, or other markings I can see. The barrel and receiver do have a pair of witness marks but nothing else. I have not removed the buttstock yet so there may be a serial number on the side of the tang, but I don't think that will tell me much. I measured the bore and chamber with a digital vernier caliper and got .4285 for the bore and .5130 for the chamber. My guess is that the caliber is .43 Spanish, but I am not sure. Should I be looking elsewhere for markings? Any idea what country this might have been made for?

Some places for a very careful look:

1) Remove the buttstock. There should be a serial number marked on the lower tang.

2) Look very carefully at the top of the barrel just in front of the receiver. There might be a faint caliber mark there.

3) Remove the forestock again. Look at the front face of the receiver, where the wood is inserted.

If none of these places produces anything, I think something must have been scrubbed.

---------- Post added at 11:26 PM ---------- Previous post was at 11:18 PM ----------

Further thoughts:

Post good, clear photos of the backsight and its markings.

There are subtle differences in the various versions and scaling of the backsight. These can be used to identify the version - or at least narrow down the possibilities.

If the rifle is worth keeping, then it will be worth your while getting
"Remington Rolling Block Military Rifles of the World" by George Layman.
which provides the best source of information on the various versions according to caliber and user nation.

I would buy some cerro safe and do a chamber cast. I have an 1867 Danish contract RB and the barrel measures something like .482 bore and .510 groove. Tight.