They may run with two order number systems the dealers and the Seat one. The dealer knows the correct one as far as Seat are concerned.
Some people use to phone up customer services without but they tightened their process and request it now. I always had to do it. Saggy slipped by without as I recall then had to get it. Stops calls from Nigeria one suspects.
I assume that once SEAT give a confirmed delivery date (which will probably change later) they will already have allocated an 'official' order reference number by then anyway, which will also have been given to the dealer
They allocate an order reference number as soon as the order is accepted by the factory. The initial dates they give are only an estimate and frequently change. Mine has changed three times from 47/16 to 5/17.
I think you only get a confirmed date when they have issued a chassis number and the order is very close to the actual build date.
Recall it's about two weeks before, for the build date to go firm from provisional to firm. That was the thing with my dealer that they were only going to tell me when the cars was built and just keep quite on the rest. Basically what you want is a provisional build date which you can get a month in from placing the order, customers services will give you that. Then phone up two weeks before that to check it's gone to firm. For that conversation and all conversations you need the reference number from your dealer else they might tell you to get it from the dealer which mine did. They might do that at anytime if you don't know it. That's their security check along with the normal.