This bulb is for the light gate (opto) of the carousel indexing system. Get two so you always have a spare!
Install using a pencil, pen, or 1/4 inch (~ 7mm) dowel and a short bit of rubber tubing (or tape) on the end that is a snug fit on the glass bulb.
Wurlitzer technical info and parts catalogs are available from this clickable link
Old part number 0007000016 or 00 07 000 016