Warning: This is a research-as-I-go comment - my thoughts. Be warned!
First thoughts: It is NOT an issue Enfield P1853. The VR on the lock plate had a dot to separate V.R. and the examiner's mark is not the standard Enfield or Birmingham one. It MAY be a Belgian rifle, but that would have Liege proofs on the barrel but this one has just an 'S' in oval. It does appear that one of the 'British' stamps on the barrel is overstamping a 'star'+letter which was a Belgian makers mark and, if this is the case, the other inspectors marks (too many for an issue weapon) are overstamping Belgian proofs.
There would also be acceptance marks for a service weapon - arrow over WD. These are on all foreign contact weapons which were fire-tested for group in batches and marked up if passed. The inspectors marks would not normally be present and never in this quantity. These marks are very suspect.
Thinking on: The butt circle is NOT the British issue one nor are the 'sold out of service' arrows (-X-) which appear to be one single stamp not the opposed single stamps normally used. Unusual, but not impossible. It has the wrong ramrod, as these type 2s had a 'tulip' type ramrod with a 'swell' to fit into the top band to hold them in. This rifle shows no sign of a ramrod spring drift pin below the lower band. These later ramrods were also wider and would not easly fit the channel in the older stocks! No British weapons were serialised like this - but Portuguese ones were. On inspection, the 'inspector's stamps' were not British.
(Were they added later to 'sell'?)
Final thoughts: So what the **** is it? I will go back on my first statement and say it MAY be a surplus issue weapon. Further research reveals that the 'V+R' and the italic date are found on FRENCH manufactured rifles - some 20,000 - contracted to the St Etienne Arsenal from 1855-58 to make up losses from the Crimean War and to replace the Minie Rifle. The 'S' in oval' actually has a crown above it, which was the proof mark for St Etienne and appears on the stock as well. The inspector's marks may well be St Etienne inspectors marks too. (Crown = Napoleon III - Emperor of France)
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These Type 2s are RARE items as those that were issued had a hard life. It MAY be one of the last batches and kept in reserve but should still have that acceptance mark . The Type 2s were gradually put into reserve with the introduction of the Type 3 in 1858 and many would have been sold off as surplus. Both sides in the Civil War bought them in from various sources. So where did the serial numbers come from? Portugal, perhaps, but also on Confederate weapons bought in. As to whether the Union forces bought them in is questionable, however, they are seen with both Union and Confererate Forces, but may well be 'war booty' and returned to issue, replacing smoothbores. Being 'sold out of service' means they would have been bought from civilian gun dealers. The ramrod may well have been an in-service or a dealer modification.
In all, darn good value for $200.