Looks like the typical M1816 conversion, if you're looking for a value I'd guess you'd want to start at about $350-400 and depending upon individual parts condition it might work up to the $700 range. A week or so ago I handled several in better condition that started in the $650 range, also handled one that was in all but arsenal new which was selling for $2200 which is the high end for this arm... I was $2200 short.
The carving in the stock is typical of the period and means nothing more than a soldiers initials, unit etc. And could as easily have been done post war in civilian hands. It looks like the typical cone in barrel conversion which would have been done at an arsenal. The 1825 on the lock is a little unusual as the majority of 1816's I've seen that were converted are post 1830 but the govt converted one in the best shape, not by year of manufacture.
The M1816 were heavily used by both sides early war and continued on in CS service a little later in the war. They were a good servicable arm, though nowhere near the quality of the M1842 which was interchangeable.