Mamiya C330 120/220 film switching issues

[NGH]

Hi
Found that my recently purchased C330 has a slight issue with recognising the film type. This is the second one of these I've owned and I am familiar with how it should work.

Today I loaded 120 film and set the plate in the back to 120 closed it up and went shooting. At shot 12 I expected it to know it was the last frame and wind on to the end of the film. Instead it went to 13 then 14 etc as it would if it were 220.

I got it home and put in a test roll I have, double checked it was set to 120 and started clicking through the shots, again it kept going all the way to 24.

I opened the back door and checked that the plate is moving correctly ; all seems correct the plate swings and the thin armature that is connected moves up and down with the plate.

It's difficult to tell whether the end of the armature engages correctly with the 'switch' in the back when the door closes, but it seems about right on initial inspection.

Before I start digging deeper (in daylight) later this week I wondered if anyone has any experience with this mechanism and if there is something in particular I should be looking for.

Thanks in advance