I only remember the lost sword was some family treasure of the shopkeeper, and he gave it to Zoro so that it would have a worthy owner, or something. It had to be pretty famous for them to make a scene out of it.

I don't think the cursed sword really is one of the 21, though. It's just equally famous otherwise, or should I say notorious. In a sense it's even more unique than the 21 meitous, of course.