Left was stock on all, parts and wiring extension existed for installing right, and at very least by '41 a few states required both sides and so got vehicles built with both. From the '28-48 book, don't have a 1940 book here, looks like '40 was first year of commercial to get the single bulb light, a version with 01C number. This is basically the same light as '33 Ford and various commercials up into the 1950's, but needs some research...you want a painted one (they exist in R&L, but the R is fairly scarce) and there are variations in mounting hardware over the many variations of the basic design.