Runck and Con-Fer were two other companies that made them for many years.
I think Con-Fer finally dropped them from their catalog in the mid-nineties, and Runck was probably gone from the backs of magazines by then already.
Might have still been around, but did not see their ads any longer.
Even Kayline and Bestop made middle seats for awhile I think. Not sure though.
Were they ever offered as a dealer-installed option by Ford in their accessory listings?
I don't remember ever seeing one from Ford literature, but I'm not that familiar with all the years of available accessories.
Paul