They were originally marketed by Pro Line in about 1971. Supposedly designed by Ron Chidgey. Southern R/C took them over at some point, then ownership past to Dave Brown when SRC closed. I have used them for decades and had a number of sets. Always worked very reliably, even when worn. Using them in both my Tiger Tail III and H9 Phoenix 7. Tony F has some excellent tips in his write-up mentioned above. Recommend ordering some spares of the small tab that your servo rod/clevis attaches to. These often seem to break in a crash, or if you accidentally extend the gear when the plane is resting in a cradle stand w/ gear up, etc. I've almost always used 2 servos on a trike setup. Always felt the convenience in field assembly was well worth the 1oz weight penalty. Mains may be a bit tall for some wing designs where the gear are near the L.E. I have FAR less gear issues in my P7 (like zero) than my fellow flyers with the electric retracts.