First of all I think that I would have gone absolutely ape**** if this had been done to my tractor!!!!!!
Back to earth, I don't think that it would be possible to fit something as big as a 14" clutch plate into a 135. In the UK from late 1970 onwards the main 11" plate was changed to a 12" plate with a different flywheel. In order to fit a 14" plate (which I don't think is possible) they would have to machine so much metal off the flywheel that it would make it impractical.
I have to say that I think that to say a clutch unit for a UK built tractor in the US is bull****.
If MF supplied UK built tractors into the US they should have been able to supply one. This must be in addition to all of the non-genuine tractor parts suppliers in the US.
I am not aware of any differences to the dual clutch units. In the UK 9/11" units were fitted and the later 10/12" ones. These were made by Spicer-Laycock.
If you haven't already paid them, DON'T. First of all confirm that it is a 14" plate that has been fitted. Find out exactly what they have done to do this. You don't say whether you have a dual plate unit so I'm assuming that it is. Here are some numbers for UK build 9/11" clutch units; 886 384M91, Bepco 30/200-1 MF3620 401M91, Bepco 30/200-1L MF 228005741.
If and its a big 'IF' they have fitted a 14" clutch it could be from a bigger tractor where the operating leverage on the unit is much greater than that for the correct 135 one. If this is the case then its no good for you.
From what you say they may have turned a dual plate type ito a single plate type where the operation of the motion clutch will also stop the PTO and hydraulics.
Would it be possible for you to post any photos of the unit taken through the inspection access plate underneath? If you can it would help a lot to determine what has been done.
Let us know what you find.
DavidP, South Wales, UK