Its quite simple at this point.
If X comes out this summer per Elon.
It will be the same schedule at the very least compared to the Model S release a few years back.
1-Spring time Signature S folks were contacted to build their cars, but no one else had access to the design studio.
2-Deliveries started in the summer
3-Test drives for reservation holders started in summer
4-Non Signature folks got to access full on design studio in the fall based on reservation number.
With Model X, I gather there will be a larger reveal by Springtime as they now have a fully operational well proven Factory-Assembly line.
Its also likely that the design studio might be available to view by more than just Signature folks this time around.
The only reason they might limit the design studio to Signature folks only... is to see what general order prefs are before they let the flood gates open for full public viewing.