Decent site link to help determine age of water heaters
http://www.fastwaterheater.com/waterheatermodels.asp
Also, I never new this or thought about it:
Electric Only
Bottom board: For electric tanks in unheated spaces, the tank must be placed on an incompressible, insulated surface with a minimum thermal resistance of R-10. This bottom board is to help prevent heat loss out of the bottom of an electric tank and can increase energy efficiency.
Makes sense to me!