this design seems ok but then they put a plactic adapter on the end...fail. on the inside of that plastic adapter you should be able to use a hex socket to remove from you extension wand. I suspect that the threading is 3/8" or possibly 1/4". You should then be able to match that up at a local pressure washer dealership. It will be 1/4" MPT x 3/4" FGH or 3/8" MPT x 3/4" FGH. Im guessing by looking at a picture of your wand only so still take this adapter out and check to verify the size. If you are having trouble finding the adapter then check here.
Pressure washer parts catalog quick links Pressure Washers High Pressure...
Give them a call and they can help. The item numbers you are looking for are
1/4" MPT x 3/4" FGH Item # 3250
3/8" MPT x 3/4" FGH Item #3251