Knight Lights is correct. In addition, it is a GY6.35 base. Also, the "can" which holds the socket, can be moved forward (towards head) and back by loosening an allen wrench set-screw, moving the can, then re-tightening. The design is elegant and effective. I believe that Otokoyama is responsible for the design, if not for the actual machining of these cans (although he may have done that as well).