If you haven't owned the bike since new, likely it isn't the original rotor; there have been at least two 74 mm. OD rotor widths. If you have owned the bike from new, two rotor widths were fitted in 1970 - one to triples, the other to twins and singles, your bike has always had the wrong one.
I was concerned moving rotor out 6mm meant the nut was not on the full depth.
Use one of the nuts that goes with the wider rotor?
- past Triumph dealer, currently owner of US parts wholesaler Coventry Spares - has posted previously that the most-secure rotor nut-'n'-washer are the latest parts fitted by the Co-op.
Btw, there never were "welded" rotors (magnets can't be welded, at least not if you wan't 'em to stay magnets). The rotors consist of a large hex, a wedge-shaped magnet sits on each side of the hex. and they're all held together by the material moulded around the hex. and magnets. What "tridentt150v" is alluding to is the cast material keeping hex. and magnets together comes loose from the hex., they move relative to one another during acceleration and deceleration, the otherwise-unattached (wedge-shaped) magnets are forced outwards, they don't have to extend very far beyond the rotor surface before the contact the stator ...
Btw, before worrying too much about the rotor/stator relationship, you might want to check that nothing's loose on the rotor - if it is, the rotor must be replaced, the new one can be a wide one and the axial relationship problem is also fixed (expensively).
Ideally, you need to be able to grab the rotor firmly in both hands and attempt to turn the engine with it, any movement in the rotor itself should show up/be felt.