Mk3 front master cylinder

If it has been fitted with a 13mm cylinder from AN you will see the 13mm dia piston sitting through the rubber boot, the handle bar lever bears against this. If it still has the original diameter master cylinder this will be 5/8" dia (16mm)

If it has had a sleeve kit fitted (from RGM) the piston pusher (the item that the handle bar lever bears against) is about 20mm dia and has an adjustment screw in the middle. Which i my view makes for a better arrangement as you can adjust the piston travel to the optimum position. 

Further to Ashleys comment below, take a photo of the area of the handle bar lever where it bears against the piston. The difference between the sleeve cylinders and Non sleeved cylinders is quite obvious (if an RGM kit was used)

From your comments it sounds like the bike is somewhere else getting work done, your not doing it yourself? What is the history behind the bike? There can be a lot of bullshit and slaver talked about these things and the unwary can be told all sorts of things that are 'needed' but actually aren't.

I have a friend who owns a 1906 veteran car. He had it in storage for years. When he told me about it (albet reluctantly) I said 'you want to be getting that back', 'he (the owner of the place where it was stored) will be sitting on it waiting for you to die'. Two years later i got a phone call from my friend, he launched into conversation with 'You were right' 'right about what' says I.

'That car', 'he was putting it about he had a veteran car for sale'

Upshot of this tale is that he got his car back but the 'storage owner' wanted rent for the years in storage that my friend had to pay to get his car back.    

Regards Peter  

Send a photo to me and I will be able to identify the casting, This will narrow the answer down, if it is the original casting then then could still have been sleeved sometime in its life. 

I don't think early RGM versions had the piston which could be adjusted.

On the ANIL MK3 master cylinder where customers did not like the lever travel I modified the master cylinder eliminating the need for an adjustable piston, for those wondering the MK3 master cylinder this was done long before covid, so now the MK3 levers do not travel so close the grip. 

The earlier RGM version didn't have the adjuster but it was still approx 20mm dia.

Hello Christopher,  if you're looking for someone to sort your cylinder out you could phone my friend Barry. In recent weeks he has modified several cylinders for people on the recommendation  of RGM , he's also done several of his own and those of other friends. If you'd like to phone him the number is. 07484 803644. He has asked me to get in touch so that you can phone him if you wish.

If possible take somebody with you that is experienced with commandos and their niggles.

Could save you a lot of grief and might  speed up getting it on the road.