To add to this post and elaborate a little more….this is what I have been told and obviously, there are always exceptions to the rule.
Part number 6012 was used by both General Electric(GE) and Tung-Sol(TS). This bulb is the wide flute lens. In the late 60’s it was superseded by part number 6014 with a narrow flute due to government requirements for some sort of regulation on the width of the light beam.
To add clarity to my post above the GE bulbs were stamped "Made for Ford" on the back with ink, no stencil on the front. 1966 the TS had both "Fomoco and Made For Ford" etched on the lens of the bulb in the center.
This is probably splitting hairs and could be a matter of opinion and personal findings and documentation of original cars. I have been told the GE bulbs were early 1965 through approximately April 1965(I know Bob you said March…I don’t doubt that at all.). During late 1965 and early 1966 they used both GE and TS bulbs and were all TS bulbs by October(+/-) of 1965 for the rest of 1966 year and continuing on. The embossed or “cats eye” cast in Ford logo bulb wasn’t used until the 70’s.
Another thing to mention is the bulbs had a date code cast in the lens of the bulb.
Also to note just for fun knowledge, Tung-sol was bought out by Wagner on August 19, 1966 and became Wagner/Tung-Sol for a while and then later just Wagner. After the companies merged, Wagner expanded their brand and basically took over the market making bulbs for all major auto manufacturers.
Of course, there are always exceptions to the rule, this is just what I have been taught and I thought it might be helpful to share. I do not intend to take anything away from the post, just wanted to add a few things to help further the discussion. Please don't take my post the wrong way. Thank you.
I hope this helps.