I found the air adjustment plate, along with it's brass bushing and attaching screw, on the floor next to my burner in the basement. Didn't know what it was at the time. We had our annual maintenance check a week ago and I thought that perhaps the tech had removed something and forgot to put it back.
I had a technician come out and he confirmed that the "part" was the Air Adjustment Plate. The Hydraulic Jack had been removed and the official story is that sometimes the hydraulic system leaks and so standard practice is to just remove the jack and plate. (Not really necessary?) Guess the original technician just left the plate behind.
Does this make sense, or should we have a functioning air adjustment plate in our Riello??