Tom -

From the holder and the info on the letter, it appears to be a '300 K in a Season' ball.

I don't have one, but I have seen others. It falls in the same category as '500 HR', '300 W', etc, etc balls where many were done and certified for resale.

In my opinion, it is a neat piece, but not that unique and I'm pretty certain it's not a 1 of 1.

All the best -

Smitty