This is absolutely genuine, and not a reproduction or copy.
It comes with a signed certificate of authenticity from Studio-Props. We purchased this item directly from 20th Century Fox and have placed a copy of our receipt from Fox on the C.O.A. to prove it.
This is a metal branding iron. There were branding irons used several places in the film. The end of this one looks like it would leave a mark that looks like a "K".