Howdy,
I'm pretty sure the mirrors were a dealer installed option....which was pretty common even into the 60s and early 70s. Often, even if they weren't an option, they were dealer installed due to the good chance of breakage in transit.
That is why some cars have them, some don't. And you may notice that they ain't all located in the same place, nor are they always the same type of mirror. Even the old Volvos that I've had have different mirrors in different places. I'm not sure how that is treated in concours competitions though...you'd do well to do some more research on what's "proper" if that is your plan.
As for the leather straps...I don't think those were ever actually offered by the factory. The hood latch was inadequate for high speed racing, and the strap was an afterthought of a solution. If you had a 120 with an original bonnet strap, chances are there is some significant racing history that should be researched. A stock street car would never have been sold with a bonnet strap.
But maybe some of the factory rallye equipped cars would have had one as well as a bigger gas tank, two spare tires...(no boot space)....and some other stuff.
Again though....those books I mentioned earlier will tell more than I know...
Cheers,
Matt B