I'm not familiar with the styles of protectors used in the past (not on U.S. systems anyway), but it looks rather large to me for a phone line spark gap. Protectors for a normal loop were double-pole, although it's possible that this could have been an old-style "earth return" line, especially given the very rural location.
From the general appeareance though, I'd be more inclined to think that this was probably connected to a radio antenna.