We looked in more detail at the frame bolts to find that the nut had sheared off the rear bolt - the annoying thing is that the nut is not there. This means that a previous owner knew about this and sold the bike on. It also has never been discovered in the annual inspection of road worthyness (MOT in the UK).
No. On the 1999 M900’s, the rear bolt threads into a drilled and tapped portion of the frame on the clutch side. There is no motor mount nut for the rear on that bike as new.
Fun fact (joking: super annoying): The thread pitch of the rear bolt is different than the front. So, if one is NOT careful & tries to thread the front bolt into the rear mount, well, things can get bad quite quick. Nichols engine mount studs.