FS did not construct the ladder. I think there is some confusion as to why Bill left that note (my opinion is that it wasn't addressed to the original builder of the ladder, but to a recent user who had left climbing rope on it) and I think it was because he was interested in the population number of the bat colony as he had been taking counts, and mentioned that in the space of a decade it had gone down from 40,000 to 4,000.
It would be interesting to know who built that ladder and for what purpose.