The Japanese stuff is not online. The article is in the Journal of Society of Photographic Science and Engineering of Japan. You have to email (please- not pm) me since I am not going to put any more info on this here.
Also, I cannot find any reference to feedback control in Agfa's patents in the 1970 era or before.
As I described before, the technology came out of a research/science group from Mortsel and I would not be surprised if they didn't get a patent at that time.
Even if there is no implementation of a particular technique, most companies would file a patent to protect the idea in the event it is used in the future.
Agreed, but at that time the US Patent was 17 years from the date of patent issue (now it's different) and technological advancement wasn't fast enough so that the 17 years wasn't worth the trouble, unless they feared the technology would be used by others before then. (It's actually more like 20 years because people try to delay the process very much to extend the later end of patent protection.)
In reality, controlled double jet was commonly used by research communities in many corporations and universities in 1960s, so I doubt they pursued patent protection for it. It was actually good though, so that the amount of knowledge scientists learned at that time gave rise to bigger waves of technologies later.
Another thing about patent is that you don't wan't to be too specific, and the details of feedback control wouldn't be described in patents unless the inventor is very confident that other ways cannot be used. This is because the more specific you get, the more specific your right becomes (in claims, which is all that matters when you fight in court). You can describe more in the details section, and it gives you the priority of the knowledge, but won't give you the strong (offensive) rights of patent. At the same time, patent examiners don't want to give patents for something very broad and nonspecific (although they do occasionally, and this is a big pita to everyone but the assignee) so it's very difficult to write a strong patent application for a method for which the inventors do not have very clear pictures of all possible end products that can benefit from the method. Good patents are equipped with a couple of broad claims and many small specific claims. Broad claims can intimidate competitors but these are easy to get nullified in court. Specific claims are harder to get nullified. So many specific claims (dependent claims) are included to protect individual product groups separately. Some of these may get nullified but the rest are still valid and still have the full legal power within the specificity of the claims.
