theres' basically two types of mikuni main jet for the aftermarket world.
4/042 and N100.604
4/042 is common on VM18/ VM20 / VM22/ VM24 / TM24 / TM28 / TM32 etc. 4/042 is a 6mm hex jet. N100 is, well, i don't know. it's standard on a VM16 and some other jets.