The meter was reading 120V between the W pin and the ground pin, and the box is labeled 3 phase.
My guess is that this is a 208/120 wye service, and that from any of the X, Y, or W pins to ground you would see 120V, and from any X,Y, W pair 208V.
This is not necessarily a code violation. The receptacle is rated for the voltage and current. As I recall, code does not require that _standard_ receptacle patterns be used, and the 'non-interchangeable' requirement is by premises. If NEMA 14-30 receptacles are used exclusively for 3 phase receptacles at a given plant, and never also used as single phase receptacles, then this isn't a violation...even though the _standard_ would limit 14-30 receptacles to having a grounded neutral on the W pin, and say that you should use NEMA 15-30 receptacles for this application. See 406.3(F)
IMHO sticking with standard receptacle patterns is a very good design practise.
-Jon
