I had five of these WM P-51s over a ten-twelve year span, but not one at the moment. All with non-adjustable mechanical retracts/tx/rx, etc. As I remember (??) it worked if one had the servo arm slightly past center when the retracts are up. Then when the servo cycled it stopped just before it got the center, so the wires/linkages did not hit each other, just kind of parallel to one another across the servo. Kind of a fine adjustment.
Brian