It's a barrel one but non standard. The threaded portion is wider than normal and actually screws into the wood with a slotted head. Mine has done exactly the same only working if you pull the jack out a little. These Jacks are not available from Hohner any more and they advised me to get one for another type of active bass (Hohner JJ or similar). This is however too narrow to screw into the wood and too short for the length of the hole. I am considering routing out the hole on the inside to 18mm to fit the nut into. Alternatively adapting the sleeve of a female socket.
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