It goes all the way back to the slant 6 and possibly prior. They have special washers that basically act like a "spring" and allow the manifold some "wiggle room" if you will for expansion with heat.
What they should have done was put studs and nuts on instead of bolts.
Yes, the washers are like ththick Belleville washers and appeared to me to be of a hardened construction. The Chev 'liners had similar washers.
Everything engineered over the last 30 to 40 years has used metric measurements. Stepping up from the 5/16" or 8mm bolt or stud size to 9mm would be a vast increase in strength. Combine that with the conical washers and the bolt breakage would be pretty much elliminated.