Threaded 1/2x28. I'm assuming the piston is interchangeable between the dfndr9 and dfndr45, and want to add the capability to my dfndr45. I'm kicking myself for not buying the piston when they were still in business. Perhaps your dfndr9 blew up and all you have left is a piston?
I would also entertain buying your dfndr9 with piston too, if you would send the piston on payment while the can is in jail.