It's for a Mazak then, and a fairly big one, as BT50 is a relatively big spindle.

https://www.nriparts.com/products/ma...holders/752646
Not as much as what I thought it would cost, but I think that link is possibly for New Old Stock.

As I said, they're the kind of parts that are worth money to the right person.
However, you could list them for sale and somebody might pay good money for them tomorrow, but the more likely outcome is they'll still be sat gathering dust in 10 years time.

Personally, I'd stick them on eBay, ensuring you list all the part numbers, and see what happens.