The bolt that goes into the bottom of our main boom outhaul's Leroy-Sommers gearbox is a standard M8 hex bolt, coarse thread: 1.25 pitch.
Thread length 20mm, no shoulder--fully threaded. Backed by a 32mm OD fender washer and standard lock washer.
I have no idea if it is original or not, but it fits and works.
Judging by where the threads engage, it looks like the shortest standard shoulder bolt (8mm shoulder length) would be a bit too long for the washer--as installed on Harmonie.
Hope that helps!
Fort Lauderdale, FL, USA