The Z20 and Z21 stampings are date codes. 20th and 21st week of 1964. Many parts on my Nov '63 build show the 45th or 46th week of 1963. My centermost supports show Z45, the driver side support shows Z43. The date coding of sheet metal components has been discussed here in the past. As I recall, the "Z" is not well understood, I recall that the thinking is that the letter prefix may have identified the location of one of the sheetmetal fab plants around the country (I seem to recall some of my components show a "W" prefix instead of "Z"). The no. seems certain to correspond to the week of the calendar year when the component part was produced and will certainly be in advance of the final assembly.
Take a look at how the narrow sheetmetal strip that is between the windshield and the louvered vent panel is attached.
I believe this strip, which is painted body color, is part of the upper dash & duct assembly. Not a separately serviced body part that I can tell.
Looks to me that the strip is secured to the centermost supports with mastic (in the same way the hood inner structure is attached to the outer hood skin).
I suspect the "extra" support ensured alignment of this piece once made part of the upper dash & duct assembly.
That would go a long way to explaining why Fisher always provided this "extra" piece.
The attachment of the supports was by spot welds, so they wouldn't have any substantial structural purpose, but "structural" in the sense that they wanted that body color sheetmetal strip to align nicely, the supports to either side of center gave them two "solid" places to secure it with mastic.
May still have been a RH drive wiper set-up that reversed the wiper action and would have required the additional support location (as you say, they may or may not have deleted the driver side support for this set-up).
I believe the mystery of the "extra" center support for standard LH drive applications is solved.
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