Sorry if this has come up before – It’s a long thread and I haven’t read all of it:

I’ve managed to lose the adapter screw which allows the lamp to be mounted on a camera tripod. It would be easier to find a replacement if I knew exactly what to look for, i.e. if I knew the correct designation of the adapter. The thread of the central hole is (obviously) ¼ inch, but can anyone tell me what the external thread is?