Could the hex head be 5/16"? It is very close to 8mm at 7.94mm. In my experience the pitch arrived at when using a gauge is pretty spot on - until you start getting down to finer than 28TPI - it gets hard to see the threads and gauge mesh! If you gauged it as 24TPI then it has to Imperial. My vote is for Stustoys ID - 5/16" 24TPI UNF. About the tube - could it be 5/32"? Close to 4mm at 3.97mm. The tolerance for tube suggests either could be mistaken for the other.