Hi Belle, and a warm welcome to the Glass Message Board.
Your mark looks like the number "42" to me, and is exactly the right size and at the point where I would expect to see it, engraved on what, this side of the pond, we call a claret jug. The same number would have been engraved on the base or side of the stopper, and shows that the stopper has been ground in to fit this one jug perfectly.
The number was useful to stock control, warehousing, and dispatch in the glassworks for keeping the two parts together, and would have been used in the same way by the wholesaler and retailer.
Note that a pair is quite likely not to have consecutive numbers, and this can show that an order for a pair was met by choosing two well-matched ewers, jugs, decanters, or cruet bottles from the stock available, as all such hand-made items vary slightly.
I always check for matching numbers as a general or house clearance dealer may not realise the importance of matching the numbers, and get the stoppers of a pair the wrong way around.
It's a lovely and elegant item, and it would not surprise me if it was older than the Art Deco period, but that's for others with the appropriate specialist knowledge.
Bernard C.
Sorry, JP, I posted without seeing your message. Doesn't hurt to say the same thing twice.