My guess at how the process might've been done to arrive at a 1-side stitch like this: Plastic starts out as flat strip. It's overlapped onto the hardboard panel, sewed, then the plastic is heated and bent into a channel shape around the edge of the hardboard. Perhaps more clues will present themselves when the plastic is removed from the hardboard.
Reproductions
here and
here (and likely made by
REM and available thru
Dante's). Don't know if any of these repro sources would include the plastic channel, though; you'd likely have to remove it from the originals and transfer it to the new ones.