Hi Connedrod,
Speaking to a very experienced Ag pilot here and he told me the mod began to be installed in Beavers here after Harry Mangham was killed flying ZK-CGS at Tinui in 1965. Collided with fence on take off due to flap selector inadvertently left in wrong position, was distracted when farmer offered him a cup of tea between loads.
So I stick with it being a Fielder NZ mod from mid 60s.
Apparently Airwork NZ also had one that involved a twist grip on the flap handle which was also a good mod.