If I remember correctly, about 5 1/2" they used these in little planes (Pipers and such, single engine) 2-wire, you added a ground to them. Again a long time ago. I think these drew maybe 100Watts ea. Super Oscars back in the day, were $100+ea. $6 we could afford
My Father could afford the name brand lights, I was in my late teen's so I could buy these. It was kinda of a fuke, that the pilot just mentioned it to us. We the cheap (young) guy's, took it and ran with it
We really have got off track, from your first post. Huh? Glen. Tom