Voltigeur - What you have shown is an incredibly rare and desirable musket! The base pattern is a U.S. Model 1812, Type 2, as evidenced by the "stud band springs". This a difficult to find musket all by itself, much less being altered by Colt. The conversion is, as Johan correctly identified it, done by Colt. Arms entrepreneur George Law contracted with Colt to alter obsolete and condemned U.S. muskets for sale to Russia at the beginning of the Crimean War. The rear sight is correct, as it is a Russian pattern rear sight. The front sight is also correct, because the original brass blade was removed by Colt and replaced with the high iron blade you see in the photograph. The barrel was rifled by Colt using 4 narrow grooves. The original date of the musket would have been probably 1815/16. The date you see on top the breech tang is correct, because Colt polished off the old dates, and re-stamped them with the dates of when the muskets were converted.
An old acquaintance of mine, the late Howie Madaus, classed this Colt converted musket as being so rare that he estimated that there were no more than 10 examples left in existence, and that was in 1981. To the best of my knowledge, this is the ONLY example in existence of a Colt converted Model 1812, Type 2, musket, which was unknown until you shared the photographs. For which, I thank you most sincerely! All of the other known examples are converted U.S. M-1816 or M-1822 muskets.
Yes, it has been over cleaned, but Voltigeuer, if there is any way possible you can add this arm to your collection, just pay the price, it is that rare!
J.