water pump housing bolts

I have evaluated 6 big block engines around here with non-A/C setups and none are complete, most are almost there ...salvage yard engines that have been sitting for years and this is the conclusion I have come to regarding bolt length. they are a uniform bolt that are all 9/16 head wrench size.

the only part that is unknown is why, on the majority of the evaluated engines they seem to agree that a washer is not used on only two bolts ..... if anyone knows why that is...please give your input