That old tube nut does look like a metric one. I've been a mechanic for 40 odd years and don't recall seeing a mix of a square back metric type flare with an imperial threaded nut. You need to measure the thread diameter or buy an m10 x 1mm pitch tube nut. If you can screw that in then it's an m10. There is a serious risk of it popping a thread under pressure and an instant brake failure if you put a 3/8" tube nut into a 10 x 1 thread. As regards spanner sizes, a 7/16 af will happily fit on a 11mmhex nut.
We never used copper pipe as, has been said, if it's not securely clipped up it can work harden and fail. I have seen this. Cupro nickel was our favoured type. As for flares, we always refered to them as op1 and op2. If you want a concave flare to match the seating then it's op1 followed by op2. NEVER JUST OP2! I've also seen this and it was weeping fluid. I don't agree with the idea of doing both op's on everything but maybe it just makes it "idiot proof".
Thanks. I do have some M10 nuts but the nut I was trying didn't seem to want to fit into them. But I will try with some of the others, get to the bottom of this, and update this thread. Have been doing some bodywork in the meantime while I think about it :)
Yes OP2 on its own looks all kinds of wrong. I tried it on some test pieces. So it will all be either OP1 or OP1+OP2 (currently the latter).