I've not seen this thread before, so I've just got my Caithness book out, and I'm not sure about this "lamp base" at all.
It could be a decanter, cut down, or it might be shape 4083 from the Stroma & Braemar range. (p111. "Caithness Glass Loch, Heather & Peat" by Mark Hill)
It's the wrong shape completely for a lamp base.