I believe that's a version of what was called the Stainless Laminate Sporter (SLS).
The metal is actually stainless that has been blackened. They did not make a lot of these but were supposed to be the ultimate all-weather Mark V at the time.
The Grand Slam may have been a package version of the rifle.
There is another Grand Slam for sale right now on this same forum.
When the seller first listed it I think there was a Weatherby hard case with it. I believe he amended the listing and the case "disappeared"? The Grand Slam may have been a package with the rifle, case, etc.
I own an SLS and once owned another in this same 300 Wby.
I had never seen a Grand Slam until the two were listed here in the last couple of weeks.
FYI, carbon-cleaning bore cleaners will remove the blackened coating. Don't ask how I know.
My current one now wears black Cerakote.