With the serial number in the 129K range, like as not, it is a coil spring and like as not started out life as a military training musket or the same model as sold to civ's for marksmanship training.
It has been customized as was the style in the day, likely in the 1950's when nobody much gave a damn about old single shot rifles.
I have seen some prtty nice customs built on High Wall frames, in the $2500 range, pretty much set up and ready to shoot BPCR with (name brand barrel, good quality sights, nice wood, etc.) and have seen bare actions as low as $450-$550. So I would consider those two points as some pretty broad limits to work between, unless you can come up with a known name for who did the work, etc.
So somewhere between those numbers.
The later actions that were intended for Muskets apparently did not get the same heat treating that the heavier caliber and earlier guns got, apparently for cost savings at manufacture. Not that it makes a lot of difference, as the action is hell for strong, but it just was.
It is not impossible that this was converted to .218 at Winchester, as they had a very active and affordable custom service in the day. Aside from the barrel makings on the top, you would need to look under the fore end wood to see if there are any other markings there that may provide more info. With the fore wood off, it will be really obvious if it is a leaf spring model or not, as the leaf spring is anchored on the barrel.
Some clear, well lit, close-ups would help a lot to see condition. Use a tripod if you have one, otherwise play with the camera, take lots of pictures, and choose the very best ones to post.
Cheers
Trev