I was sent this picture and asked to identify it. You have my guess, but I am just not up on a lot of the old things we have around, especially when they are no longer in use. I assume that the large wire (is a wire and) goes to a grounding electrode of some sort. If you know what we are looking at for sure let's hear it.
Background: This is located on the exterior of a small (1 room) guard station built the summer of 1941. Guard stations are typically a cabin for forest personnel to use, about a 1 day horse back ride from any others. Though this building is called a guard station, it was actually used for fire control and had a lookout located nearby. I do not think I have ever been to this site, but typically there is evidence a phone line was run overhead through trees up the mountain from who knows where and terminated at the lookouts living quarters. The terminations that I have seen have varied a lot from this site and though I have seen parts of this style before, this is the most complete. These phone lines were used for reporting fire and were abandoned and most removed once radio came into use (probably due to unpleasant shocks received while using the phone).
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.