Small numbers printed to the right of the model number
overall: 1 3/8 in x 1 3/8 in x 3 1/2 in
Dekatrons formed the first generation of integral vacuum tube counter tubes. The basic topology was invented by Ericsson for telecom applications (rotary dial sensing, for e.g.) and soon various tube manufacturers offered similar units. Typical applications: test equipment (frequency counters, digital voltmeters), and in computer systems.