For questions like this, a good set of reasonably close up photographs are essential with such an aged instrument, since condition is going to be a big part of the price, and old wooden horns usually don't age well.
In any event, "worth" is a relative term here, since you are interested in retaining it. Assuming that it's a Klosé/Boehm instrument, with a complement of six rings, it's one that's still contemporary, as far as playing technique is concerned, so any method that you might utilize to learn to play would work just fine. So, you'd have a useful clarinet in any circumstance that you might want to consider it (as long as it's not cracked or otherwise workable).
However, the extra "key" under the "C/G key" is puzzling. There's a "C/F key" on the lower joint, but no touchpiece anywhere on a modern clarinet would control the note C in one register and G in the other.
Extra touch pieces (which operate the keys, the platters that actually close the holes on the instrument) on Klosé/Boehm horns are normally located on the left side (from the player's perspective) between two of the levers for the keys operated by the LH little finger (the auxiliary Eb/Ab), on the right side below the lowest touchpiece for the RH little finger (that operates the Eb extension key, only found on the so-called "full Boehm" clarinet), a "sliver" touch piece between the first and second ring on the lower joint (the touchpiece for trilling the G#/C# key, located between the two joints), and an extra ring for the LH third finger, which enables the fork Bb/Eb operation of the normal Klosé/Boehm keys. Others exist, but are so rare as to be unknown to most players.
If it's a Albert system horn (which seems doubtful, given your touchpiece counting), all bets are off. You can play Albert horns; I used to do so on a monthly basis, just to keep my hand in with the funky Albert way of doing things. However, learning to play (
adequately play, that is) an Albert instrument requires different techniques with regards to the little fingers, and I am currently only aware of one available method (the Lazarus one) that has exercises designed to wrap your head around Albert little finger technique. And, "currently" there means some twenty years ago; that's the last time that I shopped for a copy of the yellow jacketed Lazarus method.
Selling it would be another matter. An "obsolete" instrument (as most with a wrap-around register key would be considered) isn't going to be worth squat to most players, and it doesn't sound like a collector magnet. Again, good photos would help to make this determination.
You have done some of the work here already; we know the name of the maker (which is not familiar to me, by the way), the pitch (indicated on the horn in the European way by the letter "B" (which does not refer to B natural, by the way; I've always thought that the Euro "B" mark has been responsible for the general belief that B natural clarinets were once made), and by the "LP", for "Low Pitch", essential for an instrument that which will be used for playing with modern musical groups; a high pitch horn just can't be made to play in tune with modern standards).
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