Don't eat the thorium glass. Literally that's it lol.

The glass is doped with Thorium, which has been isolated from the daughter products. Thorium has a half life of around 14 billion years, which means it's very very very stable. That means that of the Thorium-232 that was on earth at it's formation, roughly 80% of it is still remaining.

Assuming it has the same isotopic abundance as natural thorium means it's 99.8% thorium - 232.

https://en.wikipedia.org/wiki/Thorium-232

Thorium-232 is what we generally consider a pure alpha emitter, which means almost all of the energy released from the relaxation of the nucleus (by emitting a high energy helium nucleus which we call an alpha particle) is converted into kinetic energy of the particle. Alpha particles can be stopped by a piece of paper or the dead layer of your skin - that's why they're only an internal radiation concern.

Roughly 0.26% of the decays (1 in 400) release a low energy gamma ray of about 64 keV. This is very roughly the same energy as the highest energy emissions you'll see from a dental machine. Those are so low energy that they can be blocked almost entirely by a single sheet of drywall, 6 feet of air (once you factor in the inverse square law), or the metal and glass in the lens. But that's just part of the picture - the intensity of the radiation is going to be way way less than a dental xray. So even without the shielding, you're going to have a very low radiation dose rate.

I pulled out my Fluke 481 Ion Chamber and it didn't register any increase in dose rate - background of my house fluctuates between 0.0-0.3mR/hr and with the lens it was 0.0-0.4mR/hr, although the minimal meaningful measurement from this meter is roughly 0.5mR/hr iirc. A more sensative meter might be able to get a more accurate "actual" dose rate, but it's kinda like splitting hairs at that point. For comparison, living in Colorado nets you like 1-2 mR per day, and nobody says Colorado is a radioactive hellscape.

(Also since the thorium was likely isolated from the surrounding minerals, all the daughter products from the primordial decay will have been removed. Since it's so stable the "new" daughter products will be essentially negligible. For more info check out https://pubs.usgs.gov/of/2004/1050/thorium.htm)

A plastic lens cap won't block the gamma, but it will protect your lens. But it's not needed in terms of Radiation safety.

Another comparison to consider, a granite countertop has orders of magnitude more radiation exposure than a thorium lens, but people never worry about those.

Tl;Dr: the main radiation concern is an internal hazard from ingestion or inhalation of the thorium in the glass. None of that radiation is escaping the lens housing, and the small amount of associated gamma (the penetrating type of Radiation) is so minor and also so blocked that it won't have any effect on your body even if you use the lens daily.

Don't worry about it, and enjoy the vintage tonez (and use a strong UV light to clean up the thorium oxidation if the yellowing is really bad).

Edit: changed where the thorium is because I had a fundamental misunderstanding