I want to assemble a pigtale -- one end will be a standard 125V female that will run a temporary lighting system and the other end a Hubble twist lock male that will connect to a HBL7310B (Hubbell 125V 20A ungrounded twist lock receptacle). The Hubbell receptacle is hooked to a generator which is on a maintenance vehicle. I have all the parts. My question is: how do Hot, Neutral, Ground on the female plug mate to the X,Y,W terminals on the male twist lock plug?

Look at the receptacle, the wider slot will be the neutral, the ground should be in the center, the screws should be, lighter colored screw neutral, the darker colored screw hot, the green ground.

In this link, it looks like the W is Ground, then like above white screw is neutral, dark screw is hot.
Product Datasheet -- HBL7310B (http://www.hubbellcatalog.com/wiring/section-b-datasheet.asp?FAM=Locking_Devices&PN=HBL7310B)

W is the neutral. X & Y are the hots. There is NO ground.

The genny frame is likely bonded to the neutral of the circuit. If you can verify this you should attach the ground from the pigtail to the genny frame.

Stan,
Since this is 120 Volts, would it be a Hot, Neutral, and a ground. I know w to be white, but looking at diagram, it "appears" Ground is w?

No, this is a non-grounding receptacle.
If it were grounding it would be a 3-wire 250v. This is a 3-pole, 3-wire 125/250v, non-grounding.
Also, the "W" would be a "G"


Wiring Scheme - 3 Pole, 3 Wire Non-Grounding
Voltage - 125/250V AC
NEMA Configuration - Non-NEMA

http://www.hubbellcatalog.com/wiring/images/nonema_20a_r.GIF

The OP states this is a "120v" receptacle, but it is a 125/250v.
If it were a 125v receptacle it would have an X, W, & G terminals.

W is the neutral. X & Y are the hots. There is NO ground.

The genny frame is likely bonded to the neutral of the circuit. If you can verify this you should attach the ground from the pigtail to the genny frame.

Your response seems reasonable and what I thought it should be. However, I received a paid-for internet response on this saying that Neutral > W and then Ground and Hot go to X and Y. This seems wrong that the Ground would go to X or Y.

Your response seems reasonable and what I thought it should be. However, I received a paid-for internet response on this saying that Neutral > W and then Ground and Hot go to X and Y. This seems wrong that the Ground would go to X or Y.
It certainly IS wrong. I would ask for my money back!

Ground would NEVER go to X, Y or Z.