TSR made extensive use of mock-up cover image art for catalogs and advertising.
In the 1992 Spring/Summer Catalog, TSR used a mock-up image for Gamma World, 4th edition.
EDIT Nov 2019
Had a GW4 rulebook come through the shop, snapped some photos of that wonderful detachable poster map that is found inside the back cover.
Gamma World RPG at our reference site
Gamma World in stock in the shop
Gamma World 4th edition reprint and PDF available at DriveThruRPG
Aliens and the Cryptic Alliances (GAMMA WORLD)
Aftermath RPG box set – early Phoenix Games edition