What Is 811S/851S

I was about to throw away the retail box that my Liteon 811S came in when I noticed something odd. There is a sticker attached to the box which says a model number of 811S/851S. I know what 811S is. What does the 851S mean?

Thanks in advance for any help.

851S is a different model drive that has the exact same specifications (Write/Read speeds) but uses a different PUH (pickup head).

They both were available at the same time, so obviously it was cheaper/easier to just make one box for both of them. Hence the box said 811S/851S. You happened to get a 811S but could have gotten a 851S instead.