If the ballast is cast iron then it likely has nuts in the pockets. Dry the boat remove the filler. Undo the nut, pull the bolt, replace the washer. Replace the bolt if it is corroded. It will.
SS fasteners are common on cast keels in some productions / countries. We have cast keel with 22 ss bolts holding it.
What I am surprised with is the washers being two single
units rather than a sister plate. Cheapo.
No matter what the reason, you want those washers out of the system. Consider using a sister plate too. Consider sealing the top with sika when driving new bolts.
The ballast is UNLIKELY lead. In lead ballast what you see in the bilge
are nuts, not bolts.