All stock monsters (except the S4Rs & S4Rt, only cuz I haven't measured these myself yet to confirm) have 54/50mm bottom/top at the triples.
The reason that OnCycles has those 53mm top triples is because it's a 'fairly' common mod to put SBK fork legs on a monster and the SBK's are 53/53mm bottom/top. <-- I did this mod myself
So if you want clip ons for your stock fork legs then you need 50mm. If you want that top triple then you need SBK legs and 53mm clip ons and a.5mm shim for the lowers.
HTH