Interesting. I've seen the occasional offset control lock set on non-UK-registered Daks but can't recall any CAA AD on the subject. Was it cancelled? There
was a 1940 Douglas Service Bulletin (# 187/187S) addressing the possibility of inadvertent take-off with locked controls whereby it was suggested (impracticably!) that the locks be attached to ground rings by cables that would pull them free when the aircraft taxi-ed forward or 10-30lb loose weights to do the same. Whatever happened to full & free checks?
Avionic type - the autopilot servo unit wasn't really much good for other than light' damping' because of the primary flying control cable stretch; it wasn't a positive lock. I forget what the A/P overpower valve setting was- 140lb" comes to mind - and the limit stops/structure would have been bent by a good gust on the large control surfaces long before the servo unit disengaged.
I'm sure you remember those follow-up cable disconnect bullets as well - a real pain to sort out once the A/P control unit pre-tensioned pulleys had whipped them forward and you needed to remove the nose panels and hatch to sort it all out.
One good modification I've seen on N-reg DC3s was the reduced span flaps that allowed un-handed aileron control locks to be fitted. There were more than a few outboard flaps damaged on extension by wrongly installed ground locks.
My habit after an engineering pre-flight check was to lay out all the control locks, gear pins and pitot cover just inside the freight door in their relative positions so that the pilots could see instantly that everything was free.
Slightly off the main topic - a South African pilot told me that he once took off from an unmanned airstrip in an Islander with the aileron lock fitted. A rudder-only and differential engine power circuit got him down to rectify the situation.
.