We have gone through the headlight learnings at work. I wish I could show you an updated listing on our catalog, but we haven't updated yet.
For the four headlight systems, I believe that the original headlamp bulb manufacturer was primarily GE or General Electric. However, when the powers that be, decided to get the Ford licensing to do reproduction "FoMoCo" scripted headlamps, GE was not interested. So I think it is Westinghouse bulbs that have the licensed "FoMoCo" script.
We were recently made aware that non halogen, round, sealded beam headlamp bulbs, are basically obsolete. All of our suppliers only carry halogen style headlamps. This is a good news bad news thing. Halogen = brighter headlights, but they also are of a higher wattage, and exceed most of the vintage headlamp switches.
It was discovered that there are two different halogen bulbs, one high and one medium wattage. The mediums are what we will be stocking for general replacement bulbs.
We do not carry the single headlights for the Mustang, our Cougars use the quad headlight system and they are slightly smaller in diameter.
A secret amongst some of us that treasure hunt in auto salvage yards:
Always check the late 1960's Ford Econoline vans, they use the correct 6012? bulb. They have fairly high positioned bulb placement and occasionally the original "FoMoCo" scripted bulbs can be found. These vans have about the highest probability for finding original bulbs of anything that I can suggest. For those of us with the quad headlamp system like on a Cougar, most any appropriate year Ford/Mercury/Lincoln with hidden headlights often have one or more original working headlights.