The slip-on hubs with a roll type drive pin were made by Ret Bar. The made them in 28 or 36 hole. Schwinn's T&C were 24" wheels so they were 28 hole.
The tribikes were basically broken down into two groups. They were either purpose built with the drive integrated into the frame design. Or they were "conversion kits" where you took the rear wheel off of a "usually girls" frame bicycle, added the kit, and an extra front wheel to build a tribike.
The two big players in the industry were Allen Gobby, Gobby MFG. located behind the Cystal Ice House in Glendale, AZ. The other player was Ret Bar Mfg. located in El Mirage, AZ. I'm really not sure what the relationship was between the two companies because they were less than five miles apart. Maybe a partnership break-up? Both were close to Sun City, AZ which is a mega retirement area with lots of OLD people interested in active retirement, "or potential Tri Bike Customers". Ret Bar was the vendor to Schwinn, and Gobby was the vendor that sold to Sears, Robuck.
Ret Bar built the kits for Schwinn. They made them in 20", 24" and 26" wheel sizes, Schwinn used their 24" wheel size kit.
I think I remember that they both (?) offered the option of the differential drive on their "purpose-built tri bikes". Sorry a little fuzzy on this. IMO the differential drive was kind of a gimmick. It's sounds good selling on the showroom floor, but it does not improve stability (tri bikes still fall over), and since it's an "open differential" it does not improve traction. It's sole advantage was the pedal to wheel speed differential was the same on both righthand and lefthand turns.
The shorter the wheelbase compared to the width of the rear wheels makes the bikes more stable to falling over. So the purpose-built framed bikes were a better ride.
John