I do not have the exactly information but I think they were hardly used in domestic radios, more like for professional/military instruments. Domestic stuff usually used to use some small tubes and not transmission ones. Transmissition ones usually were used to actually transmit signals like TV, radars etc. (some are enourmous)
The STC4242 basically is a 211 with Mo anode. These days
ELRog produce the
ER242C which is clone of the old Western Electric 242C with Mo plate.
IDK if Mo is toxic, but as someone could expect 'Mercury Arc Rectifers' have a lot of mercury. The one on that video should have about 2Kg of mercury.
Same for vapor mercury rectifiers, but they are small and the amount of mercury is not that high. However both are very dangerous to use
IF you do not know how to work with them. For instance, vapor mercury rectifiers (MACs are different beasts) need to be pre-heated (up to 24hs with not used for very long time) before the use (when you apply the high voltage) otherwise they can easily explode (because the mercury become liquid again and cause a short-circuit).
Those mercury things can handle current with thousands of volts and very stable, often more stable than modern transistors.
EXAMPLE.
In the past, when they were the thing, they often exploded because they were used near the limits and sometimes got overloaded (they were installed in cages). These days vapor mercury reciters are mostly used on valve amplifiers power supplies and far from their limits, usually because they are cool (they are electrically noisy and need to be filtered).
Mercury arc rectifiers were used when high current were needed like to feed trains. Many metros, trains, etc substations around the world still have some running for educational purposes and because people who work with them won't let them go. They can easy last more than 100 years working non-stop - but they are
crazy expensive to run because they are very inefficient.
There were some small MACs used for other purposes, like charge batteries.
Another interesting fact is anywhere where radiation is present and you need electronics valve is your only option, since transistors die pretty fast when exposed to radiation. When CERN was in construction they bought all EC8020 (a very special small tube) they were able to find. These days if you find NOS EC8020 you probably will need to pay about US$2K for each one.