I’ve learned that back in the day there were a few different manufacterers of under dash units that dealers would choose from. The Mark 4 was made by John E Mitchell Co. out of Dallas Texas. I’ve tried to research them but came up with very little. Vintage Air is selling a reproduction of the Mark 4 but I couldn’t learn anything about years or models. I have seen a photo of an older version of the Mark 4 and it had round vents on it. I’m thinking that may be the one I need but I haven’t found any literature or seen one for sale anywhere.
Thanks for the leads so far. Please keep them coming and if I “crack the code” I’ll be sure to share the news with you.