Fresh carnage!

The head style of that bolt is a clue that means it's a self locking bolt, no washer allowed. Post a pic of bottom of bolt head to show the relief groove below the head, which helps it to clench the bolt & lock it. At least when they are new.
with older bolts I might also use loctite to be extra sure as they are 50ish years old. And they're hard, too , and probably get brittle, so red loctite & less torque may be called for.
I am doing the .040 hole, which is the size of many oil feed jets on turbos, so not much of a pressure bleedoff.