Just looked at the link, and it's similar. The best tipoff for you is the metal braid shield on the pickup lead, which is not exclusive to Gibson, but typical of a lot of their pickups.
The numbers on the cover won't help you (I think) because they would be mold numbers for making the cover itself. You'd want to get it cheap to sell it cheap, because you don't know for sure what you have. I'm thinking most won't pay a lot for an unknown pickup.
Nothing like having an old Gibson employee that built them comment on it.
I built guitars, not pickups.

And I was at Gibson in the late-90's up to 2000, so what I know of their products is for things from that timeframe mostly.
The humbucker-style mounting ears on that "Gibson P-90" bother me. You may want to ask the seller what specific model it was, or what guitars it went into. Assuming the best of the seller, maybe the pickup is meant for installing a P-90 into a guitar already routed for humbuckers, and using humbucker mounting rings.
I say that because as a rule, all Les Paul style guitars used soapbar P-90's with no mounting ears, because the low clearance of strings-to-body require the pickup to be mounted into the guitar. The dogear style P-90's went on ES-style guitars and sat on the surface of the body, because the higher clearance afforded by the neck-to-body attachment and/or the bridges with the wood base and their trapeze tailpieces.
Another way of saying that is the Gibsons that transitioned from being acoustic guitars to electric guitars tended to get the dogear P-90. Look up the Gibson L-7 vs the electric ES-300 for an example of this.
So what I'm saying is there was no place for those mounting ears to be hidden on a guitar than came stock with P-90's, the way there is a routed place on a guitar that came stock with humbuckers. Look at the picture Simon posted earlier of the mini-humbucker: the guitar is routed for a P-90 (which would be the soapbar style), and the body of that pickup would fill the entire space. The mini humbucker was built to allow the installation of a humbucker into a P-90 route without altering the guitar, and the cream-colored "surround" of the mini humbucker is actually a soapbar P-90 cover which is cut out to allow mounting of the humbucker.
So I guess that's the really long way to say I think this P-90 (if from Gibson) never come as original equipment in any Gibson guitar, but may have been intended to go into a Les Paul style guitar already routed for humbuckers, as a later replacement/conversion option.