GT4 wiper arms and blades

I did a search and couldn't really find the answer other than they are not supposed to be black, I knew that already.
Years ago one of my wiper arms failed at the point where it attaches to the drive spindle and I "temporarily" used an Audi wiper arm that I had laying around just to keep from being pulled over for a "missing" wiper (don't drive in the rain anyway). I am ready to replace it and the other one which was apparently the wrong part (for RHD?) as the bend at the end of the arm was in the wrong direction causing the blade to point somewhat upwards when parked as apposed to being parallel to the bottom of the windshield. I was able to bend it to the correct angle soon after I acquired the car but if I am replacing one why not both.
My arms are a brushed silver finish with the little flip up caps covering the attachment point, same finish on the blade holder. Most suppliers list arms as "long" (440mm) an "short" (400mm) but as for the blades themselves there seems to be some discrepancy over length. Some list one @ 18" and one @ 15" while others list both @ 18" or both 15". I'd like to acquire the correct ones for my car if possible, I understand that they may be NLA but at least find a replacement of proper dimensions. Any guidance on what is "correct"?
My car is a 1977 US spec version.
mike

 

I finally found my wiper arms and blades packed away in cold storage, I have stuff stashed in about 5 locations!

I found both wiper blades are 18" long and both wiper arms are 18" long. My passenger side arm appears to be for a RHD car as the kink at the end is angled opposite of my driver's side arm which positions the blade parallel to the bottom of the windshield as it should be. I had bent my passenger side arm kink enough to keep that wiper blade from parking partially below the bottom of the windshield (onto the paint). The blade's erroneous position is obvious in the photo as the car came to me. One can see that the ends of the arms splay in opposite directions.

As for the mounting end of the arms both have the flip up cap hiding the retaining nut. On my driver's side arm the lug that attaches to the drive spindle had broken (I don't remember how it happened, it was 15-20 years ago) and coincidentally a spare wiper arm from one of my Audi's at least bolted in place as a "temporary" fix. I haven't found any replacement arms that come with the cap, only arms with the exposed mounting nut. I might try to put the old cap on a new non-cap arm, IMHO they look better.
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I haven't seen anyone listing a 20" arm, only 15" and 18", my 18" arms are more accurately around 17.25" depending on what one measures to. I guess the wiper department at Ferrari measures wiper arms in the same manner as the horsepower department rates horsepower.

Looking at your photo the driver's side wiper blade overlaps the passenger side drive spindle by the same amount as does my car indicating that both our cars have the same driver's side arm and blade. From what I have read the 15" arm (passenger side only) was a series 1 configuration that changed to a 18" arm sometime during the run, series 2 and 3 all had both arms at 18". My car (13834) is a series 2 built in OCT 1977 (black on black originally, I'd love to go back to it but I no longer have access to a free body shop).