The cars left Italy with them installed. The overriders have nicely punched holes for the mounting of the bumper bar. See the attachment from the part manual.I don't believe Alfa supplied any of these bars as original equipment. As I remember, they were made by AAMCO as an accessory and often installed by dealers as an additional option. The front center piece was original equipment.
This was only found on the US spec 1750s. Yes, there is only one bar. It's a genuine Alfa part.That's so interesting. I'm still not convinced. The parts image shows just one bar. Parts images are pretty accurate. I stand corrected that there is an Alfa rear center bar, but as shown, I have never seen it. As a part number starting with 10503, it must have been available on the 1600 Spider. I have never seen one that I can remember on either S1 model. Nor do I remember the illustrated license plate backing plate on S1 spiders. I don't think the upper crimp of the body mounted plate holder would allow such a plate, but that's conjecture. Maybe these items were regionally ordered. Maybe my memory is just bad.
The AMCO bar looks like the bar on the ivory spider.
That is a very nice NOS part. I have a very nice used one on my car. Most of the used ones have some damage. I have a spare that we professionally repaired. There are ten hours of labor in it.