Not sure if the installation instructions with the Powerflex bushes have been updated, but looking at a recent set it says to install the larger lip of the rear bush against the radiussed edge of the wishbone.
This means that one bush has the larger lip on the top side, the other has it on the bottom as that is how the cast wishbones have the radius.
Just that any pictures I've seen of poly'd LCR wishbones have them both done the same way.
Anybody got a new set to compare?