Does it ever show how did Guts aquire that first Behelit he has? It's Dark Behelit I think, and I belive it first appeared in vol15 when that elf got it out of Guts' pocket to show it to Jill....
Re: How did Guts get Dark Behelit (or whatever it's called)?
Guts gets it from the Snake Baron in the anime. In the manga, he gets it from Vargas, that stole it from the Snail Count. Guts later killed the Count, but not before this one used his beherit again, without success.