Hmmmm! Now I don't remember!!! I am thinking that this all comes out the end with the nut!!!!!
Give me some time to dig up old pictures........
DON'T PULL ON ANYTHING YET!
Look for pic's of blown out Pantera cylinder for clues. Same or darn similar!!!! 23mm also!
Edit:
OK after thinking about this some more, and unable to locate my old M/C hard copy prints on the computer....ugh.... I believe that the snap ring on the plunger end comes out, and then the large washer which simply exposes the end of the piston. Normally there was a large black rubber seal over the plunger shaft and fit into the recess also, covering the snap ring and big washer....but not retained by anything....but friction. I don't believe that these seals are available anywhere....
The large nut will be an issue. You may need to walk it into a machine shop where they can clamp the M/C securely in a proper precision vice, then using a 6 point socket of the proper size and a breaker bar to try and pop it loose. I don't think there is a rubber o-ring on it....but rather a brass-ish washer......to seal it up.
Heat may be required to heat up the casting to cause it to expand a tad. You might try that. A ship may use an inductive heater to warm up only the area you need warmed...... anti-sieze on threads upon assembly!
If your outside temp is in the single digits, you may want to bring the master cylinder indoors and set it next to the fire.......! Work on it on a warmer day?! 
Steve