One of those eBay finds from about a month ago... I have seen a number of varieties of UK third rail insulators, but I've never seen a glass example. Most are short, stubby porcelain mushrooms with iron caps. This one looks like Pilkington/Armourlite glass, based on the texture and that distinctive lime green tint. It's also a heavy critter made out of very thick glass, and likely because of that survived the trip from the UK despite rather poor packaging - it arrived in a box with only a layer of cardboard around it. No padding.
The pin hole, if you could call it that, is very shallow - maybe an inch deep. I suspect it originally just sat on a peg, and the weight of the rail was largely responsible for keeping it in place.
And yes, one of its previous owners painted the cap gold. It's severely pitted, I suspect as a combination of the UK's generally wet weather and corrosion caused by current leakage. It's gaudy, but it's grown on me since having it sitting out.