Then managed to drop the lower bolt into the blackness below, oh no.
At this point I decided to try the starter with just the top nut for now.
It must have been at slight angle because of no lower fastener and that caused it to lock up when it was energized.
Found the the bolt under the dozer and installed it and the starter fired right up

It is another old unit and has a limited life ahead so I took the two other starters to a rebuilder who will reman the better of the two.
Meantime, since the dozer was once again running, I hauled in the rest of the pine pulp I had bunched nearby and got the pulp load ready to truck for next week.