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.