Lift the rubber boots off from the bottom of the master cylinders and measure the bores directly.

You can remove the circlip retaining the inner gubbins and compare them directly.

So in the top pic which is the brake and which is clutch?


How to tell without knowing what bore size is inside?

Yes, some of this type MC have the bore marked on the outside and some do not. I can't remember if Lockheed or PBR.

What John said re putting a bit of wire in, that will tell you if there is a restrictor valve in there (used for brakes, not clutch). But this valve can be put into either size cylinder by anybody...

Some Clubbies (I think?) came out with a 0.70" clutch, but as I said they are marginal for clutch throw once things start to wear. A 0.75" one works much better.

I am sorry If I did not understand Your question, I thought You wanted to Know
Which master Cylinder is set up for brake, and which one was for clutch so that You fit
them in the correct position I did not read it that You wanted to Know the bore size,

now days You cannot take the markings on the outside .7 or .75
as being gospel I have seen many a .7 converted to a .75
by sleeving them and the same goes the other way around

Also looking at the Photo with the master cylinders fitted.
and the Doc may be better able to advise You, But it is my opinion
that the pipes fitted in that photo are incorrect as in not the correct bends
in the correct places
I am however not saying they will not work as they are.