Hey M,
I had not noticed your message from January
Your theory's correct, the body of the jug is by VSL. I don't think it has been altered but a customised order. It is illustrated in the 1908 catalogue as a decanter without the silver mounting and is called 'Sabine', shape and pattern both have this name. Normally the neck would have been bit longer and with stopper it would have been available in two sizes, 32,5 cm and 25 cm. They probably had to shorten the neck otherwise the handle would have ended up too high on this piece, it would have been too heavy at the bottom, making using it awkward and difficult to incline to pour whatever it was filled with.
I have learned recently that at VSL at the time anything was possible, different patterns on different models, anything could be discussed and customised and this is one of the reasons that a lot is not catalogued and as this jug was made to order, it wouldn't have been catalogued anyway. I have often wondered why a lot is not documented but this makes sense. Imagine being invited at a high society dinner party at the time. Your crystal glassware was a statement and you didn't want the same as mr and mrs X ... you wanted something even better and more impressive, just to show off and show everybody how well-off you were. VSL, amongst other i guess, played in on this and basically created an 'options menu', from which you could choose and actually, that still excists today. The finished product wasn't and isn't just that, it can be personalised and altered to any requirements.