The source information came from Thomas Vaehrmann , Feb 26, 2003
There have been many suggestions that Schneider Repro-Clarons and some contemporary lenses, the Rodenstock Ysarex and Voigtlander ApoLanthar, might be radioactive
"A Repro Claron was tested at a USA scientific institute to run with a geiger-mueller counter. First thought was that there could be something wrong with the measurement because the level seemed to be high. The geiger-mueller counter was re- calibrated and a measurement over three days was made
Results: 305mm Repro-Claron serial# 9956938 from 1966 gave 15000 counts per minute (mainly gamma-rays), a 135mm Rodenstock Ysarex from the sixties gave 600 counts per minute, background level was 40 counts per minute. It is the back lens of the Repro-Claron that is ho, and it's colour is yellow. The measurement is absolutely serious and indicates that the used lens in the back is not just contaminated with thorium but would consist app. 5% thorium"
The advantage of this is that in contrasty situations there will be some level of fogging, a bit like a post exposure Zone I fogging exposure to increase shadow density (Do not take this comment seriously)
However, don't carry it in your pocket or it could fade your genes - Oooops, I regularly use all three of these lenses