I suspect what we are learning - but we do not yet have any proof, is that there is a chance that the car counts for cars in the process of being retired, in the ORER's, if issued quarterly, and if submitted for publication in the ORER by the RR's based on their ICC valuation reports, could be zero to nine months out of date. (i.e. we do not know if an ORER published on January 1, is based on ICC submittals due 12/31, or the previous 6/30.)
Some roads may have gone to the effort to keep the retiring car class ORER populations more up to date than the ICC, but I kind of doubt it, because there is little or even no downside to over-counting cars in classes in the process of being retired (many of those cars may already have been on deadlines anyway - which makes an interesting point of why model cars that the ORER shows were rapidly being retired during the modelers timeframe?)
At the same time it makes complete sense that cars ordered but not yet constructed, might appear in an ORER even before they appeared on the ICC valuation report. NOT doing that would cause problems for at least some clerks, and others.
I would note that the PRR separately published an extensive list of every type of North American box car during WWII specifically to identify where those cars could not go because of low clearances. One would think that data would be updated before new cars were delivered, although the PRR's instructions to yard personnel was that if a class of car (defined by a range of numbers) was not listed in the book, PRR yard personnel needed to measure the car shortly after it was received in interchange and before it flowed through the system.
Specific to the PRR, the St. Louis to Pittsburgh line had some restricted clearances during WWII, and many cars (especially western road box cars) that interchanged to the PRR along that line were first moved north to the Chicago-Pittsburgh line, before being routed into Pittsburgh. Such cars had an Oversize placard stapled to them so yard crews and conductors could quickly identify them. Also applies to empties heading west during WWII. Interesting challenge for model railroad yard crews...
The clearance issue raises an interesting point. I wonder if one reason cars being rebuilt were assigned new numbers was because new exterior dimensions might invalidate such clearance lists if their number remained the same?
When did the modern clearance "plates" become standardized?