Hi Marty, Please stand to be surprised, These Miller sealed beams were standard items on most, if not all USA sold Vincent, after Phil's visit to the USA and meeting with Indian Sales Company in about 1949.
H4 style bulbs were mandatory on all USA sold new 1940 automobiles , so as early as 1940 these bulbs were being manufactured and supplied.
I believe Indian and Harley Davidson had also complied with this USA sealed beam law in 1940 and because of Vincent being marketed thru Indian, the sealed beam compliance was passed on to USA Vincents.
The alignment glass tits on USA headlight bulbs came into law in 1957-58, two years after USA autos introduced 12volts as the new voltage standard. So titless headlight bulbs, now 2020, for restoration on pre 1958 are at a premium.
This change to sealed beam/no park light turned the Miller switch fourth position into instruments/taillight only. So Miller switch for headlight OFF-OFF-Headlight-OFF.
General Electric Mazda 4030 bulb
Cole bakelite 3 tab plug
Miller special bucket and chrome ring