I sent it out to several people I know. The most logical answer I was given was that it is an equipment tag. Chrysler made welders, generators, and numerous other peices of equipment. Maybe the welder broke when they were doing your car and they threw the tag in the rail. Over the years I have found all kinds of things in car frame rails. The wildest one I know of happened to a buddy around 1979. He found a human finger in a rail. The car had been badly wrecked and was being used for parts.