I don't know about part numbers and such as related to what vintage they are. The later ones were in a sort of orangish and yellow package and price would have been say, 6 to 8 bucks or so. The older ones with a red, white and blue package are mid to late 70's, BiCentennial time. Anything that just looks older than that may be OK, as I have heard that from people on the forums here. In my early days, I pretty much bought what was cheapest. As I leaned more about engines and how they run and how the plug affects that, I heard about Glow Devil R/C Longs and once I experienced those, I didn't mind price too much because I knew it was a quality plug, would give me a good run, and would last the same as 3 or 4 of the cheap ones like Fox.  That was a long time ago!!
    A real Fox nut would know by the 2 volt designation,  I think. I  don't know how long they produced those, especially when the nicad ignitors came along, and they probably would light one at only 1.25 volts. If I find the energy to get downstairs ( and more importantly, get back upstairs!!) I'll check some old SIG catalogs to see what I can find in those.
   Type at you later,
    Dan McEntee

Regarding non-idle bar glowplugs...lots of people use 4-cycle glowplugs in stunt engines. I always run early Thunderbolt 4-cycle plugs in my PA .51, OS .46VF and ST G.51. I'd run them in my DS .60bb also. They look just like a regular old glowplug; no idle bar. Some 4-cycle glowplugs have an extended 360 degree shield design, but I haven't used those. You'd want to be absolutely certain that shield didn't contact the piston at TDC, but they aren't really an idle bar, either. And then, there's the OS A3 glowplug that sometimes came with OS LA engines. They work just fine.

I would note that Thunderbolt now has a glowplug called "4-cycle and Big Bore" (IIRC). Whether that is actually different than their earlier 4-cycle plugs is unknown...but I suspect they're the same exact thing. I believe Randy Smith stocks them, and he would likely know if they're same or different. I'm not into glowplug anatomy, but it'd better work!  Steve