Yes... the O.D. of the bushing sleeve. Again... may be an optical delusion. The 54mm and 58mm may be correct as the bushings were measured,,, but the powerflex website says:
"The rear lower arm outer bushing (position 12) can use either PFR27-612 which is 58mm long or PFR27-611 which is 54mm long, please measure the original bushing to order the correct part."
This is only for the 02-09 S-Type... but the PFR27-612 is standard for 98-02 S-Types. To me,,, this means that there may be a difference in the width of the ears on the bottom of the rear knuckle depending on date of manufacture... with 2002 being a crossover year.
I assume that the lower outer bushing would be the one at the knuckle... whereas the lower front inner would be the big rubber bushing. To me,,, in the pics,,, the lower front inner, (rubber), bushings appear to be slightly different in size also.