Vince, Looking at the drawing provided by Simon in post 15 shown here:
There are two valves made to the same pattern. If the stem is the same part as the disk or if they are fixed together it is a stop valve.
If the disk is not fixed to the stem and the stem only acts as a guide to center the disk then the valve is a stop check valve and it will only open if the inlet pressure is higher than the outlet and of course the stem is in the up position. The stem can only shut the valve.
The only way to tell the difference between a stop valve and a stop check valve is to look inside and check if the disk is fixed to the stem. The casting on a full size valve will have an arrow but it points the same direction on both types of valves.
On the ships we usually had a bucket of valves that needed repair. It was always the job left for when there was nothing pressing to do.
Dan