The more I look at it, with that definite taper from the top down, I think it would be for gin bottles, which were almost always tapered.
That divider probably only needed to be attached along the back, as it was likely stiff enough to not have much play. Only enough to accommodate different dimensions of mold-blown or mouth-blown tapered bottles and yet not interfere with the wonderfully tooled design on the front.
It had to be a one-off commission made for a specific customer's request.
It would certainly be a choice fashion addition to one's Texas Barbecue Gun and holster.