MYRON - Are you sure IH owned Prestolite? I scheduled batteries at FARMALL for three years... One of those "And all other assigned duty" things. First year, 1979, they came from Delco in Olatha, KS, second year, 1980, from Prestolite in Toledo, OH, then back to Delco in 1981. Those batteries were private labeled, had the IH label on them instead of the manufacturer.
I remember IH advertising Prestolite batteries as far back as the 1960's but I don't think IH had any financial interest in the co. They were just another "outside supplier" just like Delco, Taymore Ind.(wire harness') Stopler Ind. (plastic fuel tanks), and all my tire & rim suppliers, GY, BFG, Firestone, Armstrong, EWC, French & Hecht, & Motor Wheel.
I suspect the grp #26 battery was a size developed for the smaller cars when they became popular in the 1970's. According to Prestolite's website they have stopped making batteries and concentrate on heavy-duty starters & alternators for specialized applications.