Regarding Budrichard's comment, Yes, I believe many civilian defense geiger counters have a check source, to validate the geiger counter is functioning, including my CDV-700. I did check the source prior for gamma and beta readings. Regarding my comment about it being from the 60's.... I don't know that this is in fact true, as I see a web site that sells and calibrates my same detector. Though it appears they haven't changed the design, construction or components inside since the ~60's. Regardless, I'm comfortable with the accuracy of the information provided on the particular lenses I listed.
To others, if it wasn't clear, after a few minutes of research on the subject, I'm not concerned. I suspect there is some truth in what Sanguestu brought up regarding risk is with manufacturers. Interesting factoid gleened was that in addition to improved refractive properties lenses, another use that thorium is sought after a potentioal replacement for uranium nuclear reactors, due to reduced waste.
I did find it fascinating how a discussion on radioactive lenses could quickly lead to over population. Ben must have a reputation ;-)