I covered the subtle distinction between 51-54 and 55-56 designs in my gripe about the incorrect
American Arrow mirror that I got stuck with and in a follow-up posted a little further down in that thread.
You should download the
Accessories section of of the
1951-1954 Service Manual, which is archived on this site in PDF format and check out the illustrated installation instructions for that mirror.
It might help you see why the angled cut at the top of the stem, where the head attaches, only works with the stem laying in a horizontal plane, but not for 55-56, where the stem is in a more vertical orientation.
Nevertheless, it looks like both of your 51-54 mirrors have the unique attaching hardware.
The 55-56 mirror requires a formed mounting plate, which uses a completely different kind of attaching screw.
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