The Frazer Forum's been talking about tail light lenses of late. The "story" of the lights in general goes like this....
C.M. Hall of Detroit was the supplier of tail light assemblies to Kaiser-Frazer, based on K-F designs and construction specifications. The 1947-48 lights had glass lenses and there were two versions. A single bulb light was used on the Kaisers, while a 2-bulb version was used on the Frazers. The main bulb was the stop and tail light function; on the Frazers, the second bulb was for the directional signal light. Directional signals were not available from the factory as standard (or optional) on Kaisers until 1949. This is supported by factory memos and other documents released to dealers and distributors.
For the 1949 model year, the decision was made to go with plastic tail lilght lenses on the Frazer but continue to use glass on the Kaisers. I do not know if this was some kind of "test" on the part of Kaiser-Frazer. If cost was a significant factor, I would have thought that both makes would have made the switch. For the 1951 model year, Kaisers went to plastic; Frazers used plastic after 1949 for the rest of its production life.