Would appreciate anyone with an Equipment Register furnishing a Road Number for a heavyweight RPO. Did subsidiary roads have RPO's lettered for NYCS and sub-lettered for the subsidiary?
My Classification Book dated 12/1/39, revised to 12/31/43, shows several classes of "Postal" cars (NYC, B&A, and P&E) and other combination "Baggage-Mail" (NYC, P&LE) cars...
NYC Postal: 4800-4906 (renumbered from 37xx, others various MC #s)
B&A: 920-936
P&E: 901, 902
NYC Baggage-Mail: 5000, 5010, 5012, 5013, 5111-5190, 5196, 5197, 5200-5209
P&LE: 550-554
** Note NYC 5017-5020 were lightweight cars 84'6", built 1938
Hope this is useful to you...