I love a good "whatzit". It's even better if the riddle is solved, however-- and alas, I can't help with this one. It doesn't look like a battery rest to me, because (1) the glass is too fine a quality and (2) the leakage path is too short.
By (1) I mean this appears to be nice, clear, fined glass-- almost optical quality. And it has optical shapes, so: looks like a duck, quacks like a duck. Is it a duck? Fining glass is an expensive step, and unnecessary for utilitarian ware like battery rests, so it was not done. OTOH, for a glass house that ran one tank, all articles would necessarily be of the same glass. Leftover glass from a run of "nice" product would to be used to make lesser wares; what else would they do with the extra glass? (Sometimes they would cut a channel in the ground and try to stream leftover glass outside to a dump, which is a good way to burn down the establishment) If a house made a batch of colored tableware, cranberry or something special, and had extra glass, they might crank out a few insulators etc with what remained (much to the joy of later insulators collectors).
Could this be part of an optical system? Lens systems often come in several parts. Owner states it's not a very good magnifier, but not sure if that applies to one of the two parts, or the two parts combined?
By (2) I mean insulators leak current over their surface, so the shortest-path distance from the object to be insulated (battery jar, wire, etc) to the ground point (usually the bottom) is increased by the addition of ridges, sleeves, etc, which this item does not have.
Am reminded slightly of "piano insulators" (which are acoustical insulators, not electrical). Here's one of my whatzits that is embossed "SNYDER's PATT. ELECTRIC INSULATOR
" but I have no idea what it is. It strongly resembles a piano insulator, yet is clearly says ELECTRIC and has an embossed lighting bolt so that's unambiguous:
I'm dubious about battery jars etc that aren't plumb. In the age of battery jars, they were of heavy glass with lead plates, a very weighty situation, so keeping things plumb would be very important. On a ship, which is moving, tilting constantly, I'm not seeing how this would work. Might as well just make things plumb there too-- when the ship's in motion, nothing will be plumb all the time, so why not go with the simplest solution?
Could the two parts be unrelated? I've gotten two-part things before, where the two items are definitely NOT related, just that over the years whoever owned them figured out that they happened to fit, so married them. This lid on that jar, etc. The little thing fits the big thing, but that doesn't necessarily make it an original pairing.
Quite a shame about the packing. I've given up on common sense (which isn't), and people saying they will pack "well", and now I dictate how glass is to be packed and get their agreement before I pay.