Any way bleed nipples. The corrosion in the threads might not be prevented by the rubber cover which is mainly there to prevent dirt ingress during the bleeding process.
??
You have to remove the cover to use the bleed nipple.
If you look at my photo again, you'll see that the exposed part of this nipple, and the top thread are both reasonably clean and rot free. As was the hex before I tried undoing it, but you'll just have to accept that. The other end, which is at the bottom of the hole is a different matter; the plain part and edge of the seat(as is the seat in the calliper) are in very poor condition and the whole cavity packed with grot, some of which was dragged up the thread when I eventually managed to loosen the thing. The rest of the calliper internals, right back to the hose were all spotless. This was last used about 18months ago when I replaced the calliper on the other side, due to a seized handbrake lever, and bled the whole system. I have seen this before, although usually the nipple snaps off inside the calliper, and the evidence is removed by attempts to extract the part.
I find it hard to explain how this stuff ended up where it did any other way.
As for standards, this has the same M10x1 threads as any other metric brake fitting. From what I've seen there seem to be two sizes in common use, this one and another that has a thread about twice as long. It's used on a 2002 Hyundai, yet I bought the new one from a truck factor who had a box of each size, including the imperial equivalents. These are parts I replace as soon as they show signs of damage, as it's cheap insurance. I was surprised to find I didn't have any in stock.