I have changed out the bulbs in my Prime 4 bulb flasher to a number 4411 (industry number). The light works just like it did with the 74 volt bulbs in it. They are Par 36 size, just like what came out of it, but 12 volt, 35 watt flood pattern. Any auto parts store should have them, or could get them in a day or two. They are very common, since that's the bulb used in most tractor flood lights. You may want to shop around a bit; the first ones I bought I got gypped, and paid almost $10 each for them. W. W. Grainger lists them for $15!!! I found them for less than $6 each with a little hunting around.
As to getting them out of the mounts; a little trick I found is to spray the inside rim with WD40, next to the joint with the rubber, then take a Popsicle stick or some other non-metallic flat tool (I used a nylon windshield installation tool last time) and push it in between the bulb and rubber. You can then_carefully_ pry the bulb out. Once it starts out it will sort of "jump" as the rubber squeezes it over the hump, so be ready to catch it! The wires should keep it from dropping, but no sense in risking a mess. I did mine over a folded towel on the workbench. The new bulb goes in like the old one came out. Start it in at an angle and carefully push it in place, then use the stick to seat it in the groove in the rubber body.
Oh, some additional advice for anyone trying to change the bulbs: either leave the whole light assembly in the sun for a couple of hours, or heat the rubber housings that hold the bulbs with a hair drier until they're pretty darn warm. It makes it SO much easier to get the bulbs in and out! I found out the hard way not to try changing them when the rubber was cold and stiff. I ruined TWO 12 volt bulbs. DRAT! Prying on them with a nylon stick was still too much for them, and they cracked. After the rubber got hot from sitting in the sun the bulbs could almost be changed by hand, if you could ever get a finger in behind the bulb. I guess if you had the individual rubber housings out to get a grip on them you could pop the bulb out from simply twisting and pushing on the backs.
