What fitting is this?

Master cylinders over the years have always been a problem, using (sometimes) non-standard fittings. Even if they are a standard inverted flare diameter/ threads per inch, they are almost always a thread meant for a larger line size. "They" did that to prevent mixup in the front / rear lines

Over the years you can buy adapters, especially back in the day when people used brake pressure activated trailer controllers.

You have a thread gauge? I'd start there