The 439A is a trochotron, 10-line in switch tube for a decade counter or early computer
Because it is a divide-by-ten counter another application is in a clock. Two of them can be used for seconds with a Nixie clock. One could also use a pile of such tubes with a rubidium oscillator to provide the timebase. Atomic flyswatter, of course.
Early frequency counters also used such tubes.