Rod,
On lawyers wages you shouldn't need to ask this question
Just go and buy the powermate.
Steve is correct, the 2" 2X Powermate is the equivalent of the 2" 2X Big Barlow plus the Panoptic Interface. The powermate was designed from those two original components because at the time the Powermate was developed the Panoptics accounted for a very major portion of Televues' eyepiece sales.
Cheers,
John B