I took the non-working headlamp to Advance Auto Parts. Of course, you cannot look up the part by manufacturer and date--not in the database. But the clerk walked over to a shelf, picked out a box, opened it, and we examined the blades and overall size. Looked good. We then removed the old lamp from its frame and inserted the new one. Perfect fit. Sylvania
H6054. It is halogen and so may be a mismatch in light power and light color with the remaining old lamp still on the truck. But at $9.22 including tax (I was stunned), the old one can be easily replaced. I still do not know for sure whether the headlamp was burned out, but the clerk thought it was. I won't be able to find out until I solve the electrical problem which has left all of my lights and my horn without power. Oh, well.