Greetings Hendrik, I have a few of the ancient browns and love them dearly! Some are considered DZI beads as well! The problem with any stone bead is in assigning an age to it. Copies have been made since the originals! Look for wear at the ends of the beads, and in the hole itself. ALSO, the horse shoe shaped marks indicate they've been "clinked" together for a time, though this can be recreated as well, and this "mark" has many guesses as to origin and age.
Can you post a few closeups of a bead or two, and I'll give you my best guess. Anything bought in Bankok is immediately suspicious to us yanks! They get better with copies every year!
Good luck!
Jeff