Okay. Well I have an 'intermediate update.' Cycling through 0 to 70dB I can see all solenoids switching seemingly normally - apart from the second from left in the photo. Commencing with all attenuators in the 'up'/off position, It flicks 'on' okay but then remains stuck there and won't release again. And clearly its coil is remaining energized as it can't be manually flicked off, either. Not sure if this could account for the problem I've had or whether there are other things amiss (such as the perished o-ring) also.
So, anyway, once that first 20dB attenuator is switched in, it cannot be switched out again and there's a minimum of 20dB attenuation permanently in-circuit. Could there be an issue with the control logic?
Jinxie