From http://so-much-stuff.com/pdp8/repair/bulbs.php (2NOV2018)

    The official cross-reference is Chicago Miniature's CM2309, but many panels in people's collections use the equivalent Oshino OL-2. 

OL-2/CM2309

10V, 40ma, .05mscp, 5,000 hours -- The real deal. These were the "blue base" bulbs. The OL-2/CM2309 was also used in the 54-08458 peripheral indicator panel. Oshino OL-367BP is a newer part number for what appears to be the same bulb. 

CM7371

12V, 40ma, .12mscp, 10,000 hours -- approximately 36ma, .06mscp, 89,000 hours at 10V. In stock at Mouser for $9.90 for 10 (last checked 12/2011). These have been used in panels and seem to work well. The brightness is about 20% high, so you may want to do whole registers at a time to make them less noticeable.