This is a Cooper L2130PY insulated plug.  The nema configuration is L21-30.  The P stands for plug and the Y stands for yellow.  These are elastomeric rubber type plugs that fit L21-30R receptacles.  These also are made to work with the watertite connectors.  L21-30 is 30 amp 120/208V 3 phase Y.  These are made by the Arrow Hart division of Cooper Electrical and compare well to the Woodhead and Leviton Wetguard models 28W81. This one is well made to stand up to rugged industral use.  I have seen these on the internet for prices of $58-$90, and the list price is over $100, but I am selling these while supply last at $29. Shipping is $7.95 for 1, and $2 for each additional via mail or UPS ground.  (I use whichever is cheaper and will vary depending on quantity and location) These connectors are new and in original factory carton.