I have already installed and removed a 720 plenum kit on my Mk II. At present I have reinstalled the original plenum after converting the gun from a .22/700mm sniper to a .177/500mm compact.
The removed 720 part measures 11.96mm. I do not wish to disassemble the gun at this time to measure the original but I'm confident the part you are making will fit. In any case I also have 2 Mk3s I could use it on which is a distinct possibility since the Mk 2 is working pretty well in its current configuration.
I assure you I will not be making any customer complaints or requesting return/refund whatever the outcome. That a transaction like this is even possible I find quite amazing. I have no intention of complicating it further. My motivation in buying this part from you is mostly curiosity and experimentation.
Thank you for the positive gesture.
I shall also try to go to reach the maximuum that I can do for you.
If in any of your guns the 720 OEM plenum is working and the front part of plenum of 720 plenum is inserted, mine will also be inserted.
In worst case scenario, in older guns, we need to remove a slight bit of material with sand paper from the front part of plenum that is inserted into the action. This is the only difference .
Since you are alreadt using 720 plenum it means that your action cavity if perfect to receive 720 plenum front part.
So I am confident that you will not face any problem.
Bhaur