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