Simon.
I believe it is a original German valve, if you look I guess you shall find more valves with an indicator, not always identical to this one, some have a threaded ring with a tap in a pedestal slot travelling on a threaded part of the valve spindle, others have a double pointer fixed to the valve spindle, the latter you can find as a hull fuelvalve just above the IMO motor for the fwd periscope hoist on the port side of the fwd controlroom.
The main problem is that on the systemsketches we have seen you have full seapressure directly to the seawater coolingsystem without a shut off valve for diving.
Tore