Actually, as I recall*, B&O's "Sentinel Service" was not LCL. It was basically a 'tracking' service, whereby a car loaded on the B&O and consigned to a location on the B&O would be monitored, and the shipper/consignee notified if it encountered any delay. [ There would of course be an element of expediting in this.]
Hence, a boxcar with "Sentinel Service" was basically a rolling billboard, and not necessarily in that service -- but they could of course be used in LCL as well as carload service. I imagine some Sentinel customers were freight forwarders shipping LCL.
The B&O's historical society offers reprints of Sentinel Service [ and Timesaver ]manuals, I believe.
Best regards, SZ
*From reading late 40's 'Railway Age" magazines; I no longer have these.