Well Ed, that is difficult to say.

When Kodak began making dual run (salt + silver), those were in the 40s and they were controlled by hand to give identical (or nearly so) flow rates. Later a conductivity meter was added and the salt flow was adjusted to keep the needle centered. Then they found that it was a property of the halide, and they worked with pAg, but it was not accurate enough and finally they went to full control and used vAg. This process took place over 30 years, and involved designing several versions of lab and production equipment, each one better than the last.

It was a slow gradual evolution that slowed down in the 90s with the final EK models and automation. I am given to understand that Agfa never completed their automation, and IDK about Fuji or Ilford.

PE