I certainly don't question the late date.
Those are definitely NH single sheathed box cars, I'm not sure any other road had a single sheathed box car with a Howe truss and that reverse diagonal in the end panels. They were rebuilt from double sheathed cars in the late '20s (although New Haven records list them as new), and they were in non-revenue service by this time, thus not listed in the ORER.
Based on NH documents, these are the single sheathed cars I can confirm still in work service through at least April 1962:
W-1209, 1211, and 1232 (assigned Cedar Hill)
W-1233, 1535, 1536 (assigned Boston)
W-1547 (assigned East Walpole, sold 6/14/60)
There might be a few others, but the majority of the cars appear to have been condemned in the 1956-9 period, after about a decade in work service.
Modeling the New Haven Railroad 1946-1954