Is my unused AppleCare (in box) still useable?

Several years ago, I received a Macbook Pro for a new job and they also sent me a retail purchased, boxed, AppleCare "unit" to register against the MBP they gave me to use. Well, I never did use it. I no longer have the MBP (or that job) and wish to understand if this AppleCare registration is still valid to be used? I've tried many different ways of checking on the support pages, to no avail.

The product number is MA515LL/A. It says it's for "APP MB PRO/PBOOK". And again, this was not purchased in relation to a specific unit and was never registered. This is a version of AppleCare that wa sold in stores in a boxed physical state, which could then be registered to any eligible unit.

I'd like to sell it if it's still valid or at least know whether it's now worthless so I can throw it away in good conscience.

Thanks in advance.