Trying to decide if it's worth it to buy the part now.
Friday night I went ahead and took a good look at the function of the black plastic piece, and it appears to be serving as a spacer to keep the seat from rocking left to right (as in the high position the seat rubber grommet things are about 3/4 of an inch above the metal frame rails they rest on in the low position).
Under the seat, note missing black plastic seat height adjuster:
Looked around the garage, and found that a piece of radiator hose, cut to length, would slide over the rubber rests, and fill the space, I also like to imagine that maybe they will provide some sort of cushion to the ride, lol.
Took pics with only two pieces of radiator hose in place, to make it clear what was done, but placed them on all 4 rubber rest before installing seat:
And the final product:
(also stuck a couple of pieces under the irregularly shaped white plastic extensions at the rear part of the front seat, to support where they don't reach the metal frame underneath when in the high position, those are just cut to the proper length and placed into position before locking the seat down into it's latch)
Feels rock solid, no rocking or movement (if you have ever handled radiator hose you know it is very firm and supportive in short lengths, when placed on its ends).
If nothing else, perhaps this post will help anyone out who is in a bind and discovers they don't have the piece to raise their seat, and wants to raise it before ordering the factory part, or while waiting for it.