Let's pull the other v/cover to inspect. Maybe someone else can help about the Why? The original rockers are known to do this. When you build the new motor I recommend replacing all the rockers- the Mopar replacements are stronger. Even better- my '68 383 resto motor has hydraulic roller cam and roller rockers more compatible with todays oils. Even though I don't understand the Why?, I would fix the obvious stuff and put it back together. When the new motor is built, pay attention to the proper push rod length.
