Burchy, on the K0/K1 the piston in the lower fork leg doesn’t need to be removed in order to withdraw the fork stanchions - hence Honda didn’t list it as a separate part. It’s just a case of removing the circlip that sits above the fork seal.
For the later K2 onwards design the piston sits within the fork stanchion and is held captive in there - hence to remove the stanchion you unscrew the allen bolt.
Sometimes people with early fork lowers still remove piston (by removing the Allen bolt) in order to properly wash-out / clean the fork lower as part of a fork seal change / polish overhaul..
Make sense?