I can tell you it's called a "saddle top" and I would guess 1940's or newer, but there's no way to date it really that I know of. You could get an idea of the earliest it would have been made if you know when the line it came from was first strung. And if the line is gone, and you can figure out when it was taken down, that would give you the latest date it could be from
I used to collect the glass insulators back in the 1980's when the old rail lines were being ripped out around where I grew up and I still have a lot of them at my moms house I think, but I never really picked up the ceramics unless they were makers marked or had a nice look to them, yours is one I definitely would have picked up, I like the look of it.