Hey, i've a bunch of sifam knobs like that but without the flutes - also contain 3mm innards. I did ask sifam if they had 6mm adaptors that could replace them but they said nay :/
Maybe ELMA inserts will fit there?
Jaakko! pm me your address and I post a 6mm insert from ELMA to test with! If it works I know a place in Sweden where you can get them!