Mine is on my lap. #9000 is on p. 113, with a rounded sand dome. I have studied every single photo in Vol. 1, and cannot find a rectangular sand dome on any version.
Looking through Vol. 2, there are no photos showing a sand dome squared off as drastic as what is shown in this photo of a model:
That said, there are many photos of sand domes such as this...
...where the sand dome is somewhat elongated and with a bit of a flat side in order to accommodate the six sander valves mounted at the very bottom of the dome.
"The early classes of UP 4-12-2's had a smaller sand dome that was squared off. I think that was probably the 9000 (only member of class UP-1) and the second 14 locomotives (UP-2 class). The later engines had larger sand domes that were round when viewed from directly above. My Kratville/Bush book is at home--it describes the class differences in extreme detail."
I think you have things backwards. The smaller domes were round, the larger ones were as shown above.