Can't help on the date I'm afraid, but I have seen a couple of bits signed just Kosta LH that definitely were Kosta.
I have quite a lot of Kosta colora and can vouch that the signatures can vary quite a lot; kosta, kosta unik, kosta unik and a number, kosta and a number.
I suspect it was a trial piece deemed good enough to sign (and presumably sell)
Gareth