Interesting. Most Duetto B.U. light housings that I have seen look like the ones in velocewhippet's photos. The mounting points on the bumper would be at the same locations. But since housings like velocewhippet's take through screws, there are threaded inserts in the bumper. I assume that the studs on your housing require you to mount nuts & washers inside the "U" of the bumper.This what the car came with, and the holes in the bumper look factory made.
Is your Duetto an early model? Perhaps the assemblers on the line complained about having to position little nuts & washers inside the bumper (impossible with the bumper mounted) and Alfa made an engineering change to use screws + inserts instead.