Fowl pox is an interesting though. I haven’t had to deal with it so don’t know a whole lot about it. From what I understand, it would be unusual to occur only on the legs of one bird... buteo51, are any other body areas or birds affected?I just looked up some pics of fowl pox on chicken legs and that might be what your dealing with. The first article didn't mention pox on feet, but the second one does.
https://the-chicken-chick.com/fowl-pox-prevention-treatmen/
https://www.northeastwildlife.org/disease/avian-pox#:~:text=Avian pox develops slowly in,dry” form of avian pox.
Hope this helps.