Most high pressure needle valves have arrows printed on the sides for flow direction. Presumably this is a simplified way of keeping the stem packing side toward the lower pressure.
In the case of double ended sample cylinders, what would be the correct orientation?
Consider one side as the inlet and one as the outlet so both point the same direction, one into and one out of the cylinder.  This accommodates flow through and shows the direction. I assume your sample cylinders are for the transport of samples. In that case the arrows on the valves should point into the cylinder so that the valve packings are not required to seal the cylinders for transport.

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