I'm curious about the evolution of user adjustable and balanced second stages. My first experience with such a second was with Apeks which has both work-of-breathing adjustment and venturi adjustment. I then noticed several other regs like HOG are practically identical, as if they are copies. I also have a ScubaPro 109, received as a gift, although not balanced, it has adjustable WOB. I just finished watching the VDH 109 service video and heard the comment about a balanced poppet option so I looked at the ScubaPro regulator timeline on ScubaPro's site and see that the 109 was from 1969 and balanced was from 1985.
Now there was the Dacor dial-a-breath which looks like an early venturi adjust for DH regs but I don't have any idea about early venturi adjust for single hose.
Where those the first occurrences of adjustable seconds? When did venturi adjust come into single hose seconds?