R-21 Master cylinder adapter

I have a stock Master cylinder on my R-21 and don't recognize the threads for the hydraulic line. They don't taper like a pipe thread, and don't have a seat like a double flare or AN would work with. Lastly, the hole is so big an -4 male fitting just drops in. If anyone know what the threads are, I'd appreciate knowing them so that I can adapt it to my -4 clutch line.

Many Thanks.

Malc, i.d. of slave cylinder is 1.125" or 1 and 1/8" .
Thanks Frank, but I have already found an adapter.