Get a plastic syringe and a piece of clear plexi glass. Drill an 1/8" hole in the plexi. Put water in the syringe and put the plunger at a precise measurement. Fill your spark plug till it is full. Make note of where the plunger is at and find the difference. Youll need that number later. Now take the dome with a spark plug in it and the plexi on face up. Fill the syringe again and fill the dome till liquid just reaches the hole you drilled. Subtract the previous spark plug amount and that's your dome CC.
It is very possible your builder took a set of 19cc's shelf domes to cut them to 22cc'.