I have a new set of Formula R1 brakes that are of course Post mount. My fork is post mount so there is no problem there (because I am using a 160mm rotor). However in the rear my frame is IS mount. I know I would need the Shimano SM-MA-R160-P/S adapter. However I have read several places that a front adapter that adapts from IS forks to post calipers for 180 rotors is equivalent. I even see people selling the SM-MA-F180-P/S on ebay and other businesses claiming it will work for 160 rotors on the rear. If this is true, then why does Shimano have two different part numbers for the exact same part?