Not a prototype.
Metal stiffeners were used on playfields that were warped so they could use them in games.
WMS recycled pieces of cabinets in that fashion all the time. Working on a WCS 94 right now that has a back panel made out of a WCS cabinet side, also had a Congo recently with inner cabinet panels made of TOTAN reject cabinet pieces.
Prototype CFTBL pics here, from a massive restoration effort:
http://gallery.myhomegameroom.com/gallery/proto_cftbl
