4001 is the high beam, 4002 is the low beam.
Originals from at least three different suppliers, Tungsol (TS), General Electric, and Westinghouse.
I believe that the Westinghouse versions have FoMoCo cast in glass, whereas the others are etched.
Some 4001's on eBay right now - search for "fomoco 4001". I don't see any 4002's at the moment.