FWIW, my bumper had a MAAP sticker on it back in the day, truck came new from the dealer with that bumper on there, part of a Monroe Truck Equipment conversion package(RPO lists bumper delete). Seems to be stainless steel.
Pic below is from an old for sale post from another forum:
Below is some info from the web about a similar bumper:
Stainless Steel Bumpers - Mid America Automotive Products, Inc. bumpers are made from 304 stainless steel, polished to a #7 finish. The top and standard hitch pad is polypropylene, the same material used by GM, Ford and Chrysler and, is guaranteed not to fade more than 2% over a 5 year period when exposed to normal outdoor elements. All full-size pickup bumpers are V5 rated and include all the necessary mounting hardware. A stainless steel hitch pad is available in full-size bumpers.
http://www.truckoutfitters.com/accessories.cfm?cItem=04