allright, i posted this on thesamba.com aswell and got some interesting feedback.
these are plastic \"difuser\" lenses commonly fit on snowflake tail lights from '56 - '61. like ehos said, to difuse natural light (sun light) from making it look like the signal is \"on\".
eggshell tail lights are a darker convex shaped glass that fit in '55 ovals, one year only.