Height overall - 64 mm (2 1/2")
Envelope diameter - 14.8 mm ( 0.58")
Base - Miniature 4 pin, pins in a square about 7.5 mm on a side, one larger pin, 3 mm, three pins 2.4 mm.
Not visible above and engraved above the 'M.E.C. TEST' label, is 'H 1.5' which presumably indicates heater voltage.
I haven't attempted to measure characteristics or determined the pin out.