I think the sabre hidden in the rocks should be Ben’s. The one we see in the flashback in TLJ. It could be easily implied that he abandoned his sabre after destroying the temple and that Luke found it and hid it the walls of his hut. Metaphorically, Rey is repairing Luke’s mistake. Also, it’s as if the force is aware that Ben has turned and is calling for his sabre to be returned to him.
Actually, he turned that saber into the cross-guard saber that he used as Kylo. If you look closely, you can see that the hilt designs are the exact same.