Grounding on Tice Powerblock

I have a first generation Powerblock that I just changed the AC receptacles as the old ones were not holding plugs anymore. I noticed that only 1 of the receptacles is grounded, the two others are not in any way.
Any reason for this? It's been working fine, I'm just curious as to the design.
I guess no one knows the answer to this one....
Still no answer?
I had the same question when I got the Tice Powerblock 3 years ago. The answer to it might be that the other 2 receptacles were grounded through the holding skews. The receptacles Tice used is not an isolation type so the ground is connected to the center holding skew of each receptacles. Since the 3 receptacles were mounted on a metal plate, their ground were connected so one ground wire to one of the receptacles was enough.
Ground it....the Powerblock is a isolation transformer with a center shield. Grounding the Powerblock will increase the filtering of the transformer.