Hard to tell without seeing the bottom of the board but that looks like it might be a 2 or 3 section pot, highly likely with different values and tapers in each section. E.g. it might be adjusting gain at separate points in the circuit. If that's the case it will be something custom and SSL spare part is your best choice. I have had some success in the past with disassembling these custom pots and cleaning them or replacing parts with bits from similar pots of same or similar brand E.g. broken shaft