I was unaware that a bolt came in that hole from SJS. My Nomad, I do not recall and my Sherpa was bought used from someone in Canada.
On both my Sherpa and Nomad, it is an M5 bolt. If they changed to an M6, that is news to me but I have not been watching how frames are changed over time so it is quite possible that it changed.
Usually if the bolt extends all the way through the other side, it is an M6 because that is how sidepull or centerpull brakes are installed, a bigger bolt is used on brakes.
Some hardware stores in USA might not stock the right length if it is an M6 to go all the way through with a nut on the other side.
I concur with contacting SJS.