the bell housing is threaded for that bolt.
so there is no nut.
it be same diameter as lower bolt (measure it) (got caliper, no , why not?)
and has same ISO thread pitch, measure it.
that gets you to one mystery length.
you should buy a metric pitch gage for $1 and add it to your shop tool box.
and a cheap caliper so you can measure any bolt.
now that you know the diameter and pitch, all that remains is length
i take a coffee swizzle stick, jam it in the hole.
i then press thumb into stick at flange.
i extract the swizzle , lay it against, my ruler. all X inches ,the
add starter flange thickness Y
X + Y = Z
i multiply Z times ,25.5 and bingo , metric. OMG i measured length.
i then subract 2-3mm from my reading and buy that bolt.
if really flush with cash(LOL) I buy 2sizes, above calc. and next size down.
there, taught you how to fish
, no fish to give
btw, the suzuki docs dont show sizes.
and does sell the bolt. i have pn. but im sure you dont want it.