The E-Sylum hologram piece put me in mind of this slab that I'm guessing is from the late 80s - early 90s. It's 4 1/2" x 7 1/2" and about a quarter inch thick fastened with six brass rivets. These appear to have been both created and sold by SilverTowne in Winchester, IN. The reverse of the photo grade sheet describes the guarantee and buy back conditions.

I acquired this only recently, and I'm not sure why, as I don't collect either Walkers or slab types. It must have been the "I've never seen one of those before," factor. Perhaps some readers will have more information to share.