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Here's where VS was placing mirrors back in 2013 if you ordered them on both sides. If you ordered driver's side only, they placed it on the door.
I think if you started moving it aft on the fender from this position, it would be blocked by the windshield frame until you got as far back as Robert's is mounted.
These mirrors are small, and were probably intended to be mounted close to the driver (on the door). The closer to your eye, the wider the field of view.
For some reason, VS didn't want to put the passenger side mirror on the door (so, for symmetry, they put both out on the fenders). Maybe they felt the passenger's side would be too hard to see when the side curtains were in place.
Keep in mind the Porshee factory mounted no exterior mirrors on Speedsters - they were a dealer-installed option, so ended up getting mounted wherever an individual dealer felt like.
A lot of us with these small mirrors mounted WAY out there on the fender have substituted wide-angle convex glass to give a more 'normal' field of view, although even with that, the one on the passenger side is of marginal utility at best.
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