For what its worth and what I gather from silver's diagram in post #80, it looks to me..............
It is either bolts #12, which attach the fan clutch #2 through Spacer #3, through Sheave #15 (pulley) into the sheave/ fan bracket. If this is the case, I would suspect to go to a 5-bolt pattern, the fan clutch, spacer and bracket can be changed out, unless they repair with a band aide by squeezing another bolt in.
Or it is bolts #13 which attach the sheave/ fan bracket to the engine. Adding a fifth bolt to the bracket would require drilling and tapping the engine.
Another curious thing is that the Service Bulletin shown by one of the posts, indicates lack of thread depth as opposed to not enough bolts. -Hank
Gale & Hank- 2012 Berkshire 390BH