The 'K' serial number of your rifle indicates it was originally a kit (as does the browned barrel).I've never heard of TC making a convertable, but I guess it's possible.
I've been trying to gather Date material for TC's but not being able to get in touch with TCA has slowed me down greatly ... I bought a .36 seneca barrel last week, it doesn't have "seneca" stamped on it, instead it has a cross stamped on the bottom of the barrel.
It also has the smoothest bore i've ever seen in a T/C barrel.
I don't know how many times that steep downhill thing has got me, you'd think I'd remember to aim low!
I've only heard of a few of the convertables, very nice piece ya got there. I see them mentioned now and again, mostly in old catologs and mags. Your description is dead on, the drum comes off and a touchhole liner goes in, then just switch the lock to the flintlock.
I went shooting today in a snow storm, I shot 2 groups at 50 yds with 30 gr pyrodex P and t/c maxi's, one went 1.5" the other 1.125". As an example, I have two custom shop " roundball barrels.
The older one has no model name, which makes sense given that it's a custom barrel, the newer one is marked "Renegade" which makes no sense at all.
If you have a drum that can be screwed out of the barrel and replaced with a touchhole liner you have an aftermarket barrel.
Your description is dead on, the drum comes off and a touchhole liner goes in, then just switch the lock to the flintlock.
The location of barrel markings has changed over the years.
Also, don't get the idea that they have consistently added to the barrel stamp.
When you go to NAPA for parts you need to know paint color, size of lugnuts, type of seat, and gear ratio, just to get the right spark plugs.barrel with the models stamped in them are made by t/c barrels with out models stamped into them are made by other companys The thompson center first m/l barrel were made by sharon barrel company barrels made by sharon have a spade stamped into them on the underside of the barrel behind the wedge pin slot.