Hi, paddy,
I think Space Shuttle would have the flashbulbs wired in series; Williams continued to do this into the system 11 era. Fire was the first game with the lamps in parallel, requiring a higher value limiting resistance. The ones at work ate flashbulbs until I changed the resistors.
As you'd expect, with flashbulbs in series a failure of one lamp knocks out the pair, but the resistances of the bulbs is added together, so the limiting resistor (the 2R2) needn't be as high as for lamps in parallel. With 28v solenoid power being used, a pair of #89 (12v) lamps would be bright, while a pair of #63 6v would be brighter.
The #1251 bulb is meant for situations where the lamp(s) remain lit, such as Pin-Bot's eyes, and three of the rear panel lamps on Space Station