The signature is on white glossy photo paper signed in blue vis-a-vis marker.  The full slab measures ~ 10x12".

Authenticated and slabbed by Beckett Authentication Services. 

Slabbed for lifetime protection. See pics. More pics can be sent upon request.

Don’t mess around with the non-authenticated auctions when it comes to items of this value. If someone has an item to sell of this value, they will have the authenticity verified by an expert.