Steve,
Thanks for your answer. I just found the attached photo of SBD #2707 on the Internet with the following description:
"When the Seaboard System assumed the L&N in January 1983, in many cases it simply painted out the reporting marks on units and kept the number. This U23B is the former L&N #2707, which was built as Monon #608. It only kept its identity as SBD #2707 for a short period before being renumbered as SBD #3237 and, finally, as CSXT #3237. It was retired in 5/91 and scrapped."
I'm guessing that this same scenario exists for the other seven U23Bs.
George L.