Yeah, the belt rail is the riveted batten strip just below the windows, and the letterboard is the area between the tops of the windows and the roof. The coaches seem to be a real mish-mash of lettering layouts. Some had the numbers centred over the trucks, others centred lengthwise on the carbody and vertically between the sill and belt rail. These latter ones might also have the car type above the number, such as FIRST CLASS COACH, SECOND CLASS COACH, TOURIST, OR COLONIST. As with any cars in the all-green scheme, the CANADIAN NATIONAL in the letterboard might be replaced by the tilted wafer herald, centred over the trucks. In this situation, the car number appeared at the centre of the car, centred vertically below the belt line. It seems that most cars, perhaps all, with the wafer heralds did not have the CANADIAN NATIONAL in the letterboard area. The later circular heralds with the maple leaf were used only on the cars in the green and black paint scheme.
With an all-time roster of passenger cars approaching 15,000, you could probably find all sorts of variations beyond what I've outlined, too. Coach numbers seem to be all four digits, but they're all over the place, and without photos, I hesitate to cite particular ones, as there were many wooden cars still in service into the '50s. Why don't you send cn nutbar a pm asking for a few coach numbers. He has the main CNR Passenger Equipment book, while mine is only the companion volume. You may be able to borrow this book from your local library - our small-town library was able to get a copy for me from the Ottawa Public Library. The title of that book is: Canadian National Railways Passenger Equipment, by Gay Lepkey. Mister Nutbar can furnish you with the ISBN number.
Wayne