I believe some years these were brass/ bronze bolts.----Yes there is one bolt missing.-----They get very hot.----Lots of vibration.-----They come loose.-----A 1973 model 135 HP ??----What rebuilding are you doing ??
I doubt it is a 1992 model 90 HP .-----I doubt that a 1992 has those filler blocks !!!----Post more pictures of cowling / power head and cover that bolts on to cover exhaust ports.