FE/FC HUSKY/KTM Rear Brake Master Cylinder Rebuild

Hey all, wondering if any of you have had to rebuild the rear brake master cylinder on newer Huksy's (I think '15 and newer)?

Parts schematics do not show a plunger kit for this brake so I'm not sure if it can be replaced. I've searched around online and cant find anything and tried to take it apart tonight but it's definitely a different configuration than most. There is no snap ring, just a round disk with a slot in it that is too wide and shallow to fit a flat head screw driver into. Not sure if I need a special tool for this. Bike shop told me they didn't understand it and their instruction say to just replace the whole master cylinder? Doesn't make sense