I've just been for a walk with my camera.
These finials are not a rare sight around this North Manchester suburb.
I have noted the "preservation state" of telegraph poles is very regional with different rules seeming to apply. On my trips through North Wales and rural areas of Scotland there still can be seen porcelain insulators and crossmembers on poles which would have been removed decades ago around here.
So back to the finials. It's interesting to learn that they are metal, up till now I thought they were turned wood.
Note the wire going up towards the finial.
Here's a pole missing its finial, but with a pigtail of wire that looks like it was wound around the screw
and each pole had an earth rod connection of some sort, presumably the other end of the wire
So, what ever other purposes the finial provides, be it mitigating bird "guano", or protecting the end-grain, it seems to me that their principle purpose is lightning protection. I just hope Ron's example hasn't got
a radioactive isotope inside
Earth-start domestic lines (party-lines) were consigned to history by the end of the '60s around here, but it occurs to me that a linesman perched on top of the pole would need a convenient earth to test those lines.