he does, but unfortunately it would seem that his comments don't favour your suggestion that this one might have been part of a tantalus set Bernard.
Andy McConnell's book devotes two or three pages to the relationship between decanters, stoppers and the tantalus, which was a mid C19 invention - prior to which the Georgians used what they called a decanter frame (bit like an oversized cruet holder). The earlier Georgian 'frame' omitted the all important device which prevented 'surreptitious withdrawal' of the booze, and because of this the frame allowed the use of a variety of stopper shapes and sizes, and the bottles themselves might be round, square, or barrel shaped.
But for obvious reasons, the invention of the tantalus - with its top bar - restricted stopper heights, and most are some variation on the faceted ball type or other squat/square design.
There are, however, some late C19 and early C20 tantalus designs which don't incorporate a top bar, and instead use a system whereby the stopper is locked to the rim of the neck directly - yet even on these it would seem that some sort of faceted ball shaped stopper was used, rather than a spire etc.
But most importantly the answer to your suggestion that this piece might have been produced as suitable for a tantalus, is 'no' it probably wouldn't have been by reason of its shape.
'Square bottles, dating from Roman times, contain the greatest volume in the smallest space' might be one reason for the shape of bottles destined for the tantalus, but the more obvious one would be that flat contiguous surfaces conserve space and allow the tantalus to be of a more compact shape.
But whatever the reason/s all examples of the tantalus in McConnell's book show the bottles to be square (or at least rectangular) in cross section.
That's not to say that stand alone spirit bottles couldn't be round - many were - although individual bottles seem mostly to have been of some truncated shape or at the least slim barrels or pyramids, and not the zillions of other bulbous shapes designed for sherry, port, claret etc.
I'd like to express my debt for most of the above information to Andy McConnell's book - it's a real goldmine of information - not just on decanters - but on much glass history in general.
I'd recommend that you buy a copy Bernard, but don't want to be responsible for your inability to be able to pay the rent for the next six months