If it is an ASM made conversion, remember the adage, 'Buyer Bware". When I worked for Cimarron, we imported a lot of the ASM made conversions, which were also imported at one time by Navy Arms, Traditions, and EMF.....all of the importers had major problems with these guns! When I left Cimarron, they still had 135 of them that had never left the shop because they were not reliable and just down right clunkers! The cylinders were made larger in diameter, but the cylinder pin and bore height remained the same as a regular percussion gun.....what this translates to is that the bottom of the frame had to be relieved to accomodate the larger cylinder.....this left the area of the bolt/stop cut out paper thin....and I do mean THIN...I have taken and flaked chunks out of the bottom of the frames with nothing stronger than a Bic pen! Not all mind you, but enough to lead me to pass on any ASM conversion, regardless of who imported it.
At Cimarron, what we did was make a shim out of a junk bolt to sit inside the frame to support the gun's bolt. This was because when you cocked it, especially if you cocked it fast, the inertia of the cylinder would make the bolt slam into the side of it's cut out in the frame and wallow or sometimes break it away....then the bolt has all kinds of room to move sideways and the gun is a cronic 'skipper'!
You can tell pretty easy by the conversion ring itself....it has the rear sight on the top of the ring like a Richards 1860 Army did.
Another cronic problem with earlier ones was the ejector housing...the stud was soldered to the tube and was prone to break....this was the '51 ejector...later ones by NA had much larger tubes and may have corrected that problem.
339.00 isn't a bad price for a good used gun, but it would be a terribly high price for a 'kit' gun that may or may not hold up.
The one good thing I can say, is when they do shoot, they always were extremely accurate!