They used to make an aux transmission that was an inline unit, like a Hone.
But HD. Now they make OD units for either the Model A or B or both. They are still in business in Cal. Their transmissions could be OD/direct, UD/direct, or any combination. (part underdrive/part overdrive with no direct) .
Hope this helps Note; The receptionist is Mr Mitchell's wife, and VERY knowledgeable!