I found an exterior difference. Robertson has five 1/2" bolts protruding from the underside of each wing - a group of 2 in front of the outboard end of the flap and a group of 3 in front of the center of the aileron. They are there to hold the bellcranks that drive the ailerons when the flaps move. Horton does not have these.
You can see them in the left wing of this plane between the mirror and the wingtip:
http://airplanesusa.com/wp-content/u...2/IMG_7763.jpg