The SBM-20 is by far the most popular Soviet GM tube ever made. During the Cold War, the Soviets found the sensitivity and utility of the SBM-20 so good, they made millions of them as “spare parts” for when they would be needed. This is why, at one time, these tubes were so plentiful in the market. In fact, until the Fukushima nuclear disaster in 2011, these tubes were readily available. However, in more recent times, supplies have dwindled and prices have increased. It is on the way to becoming rare, and could disappear totally (or the prices skyrocket) when the next nuclear fiasco hits. But at present, it is still rated as one of best buys in Geiger tubes. Inventors who don’t have at least one of these in their Geiger tube inventory should consider grabbing a few right now, while it is still bargain priced, and before supplies dry up.  (See https://www.youtube.com/watch?v=V1p4yrVN-Cg  and https://www.youtube.com/watch?v=AIBZL0YKKBg )