Hi
@Angelo Farina If you don't have a new seat do not remove it. It cannot be flipped over. If you're definitely going to replace the seat you can screw a wood screw or cup hook into the seat and pull it out. OR try this method:
One method I've found that works rather well is to cover your tank valve with one of those plastic/rubber valve protector caps they sell at LDS. The rubber cap will serve as a seal. Poke a hole directly over the point where air exits the tank valve. Use the yoke and yoke screw to secure the seat retainer in place with the seat pointed at a child or pet. Slowly crack open the tank valve and voila!
View attachment 556857
If you decide to go that route,
@lowviz is looking for a MK15 with the hex key seat retainer.
On the other hand, I have plenty of MK10s to sell. :-D