The normal grey plastic GM Club Affinity Club card was normally the highest level slot card issued by Affinity Gaming which counts something like 15 casinos. So I would assume this sterling silver card wound have been a very elite higher level. Probably similar to the custom made high level Diamond cards from Railroad Pass in Henderson. Their engraved custom made metal cards have only been given out to 10-12 people and each are custom ordered for the individual.
Here's a link to the list of casinos under Affinity Gaming as listed on the ChipGuide
Personally I have never seen this "card" and didn't know anything about it. It would be interested to see one with someone's name actually on it. The fact that it doesn't have a name on it could mean it might have been some sort of promotional item given to high rollers maybe????
We might be able to talk to someone at corporate to get more info.