Horizon’s servo reverser is way cheap - $8.95, only half-a-buck more than their Y-cable. I thought my LHS had mis-priced it so I looked it up on the Horizon site. Yep-only $8.95. It works for me. “Expert” brand I think. The little Phillips head pot is for centering the reversed servo if the arms are set at slightly different angles when placed in the proper “splined” positions. The pot turned too easy for my taste (concerned about vibration) so I placed a little threadlocker in there (the kind they use on PC board screws) but a dot of nail polish would probably work also. Aside from hooking them up wrong, servo arm angle and push rod angle will cause the differential in elevator travel.