Joel Holmes wrote:
"The B&O placed their first Wagon Top Box Cars into service in 1915."
Completely untrue. Who is telling you this folklore? Do not confuse the
original build dates of the single M-13 or the M-15 subclass double-sheathed
boxcars with the dates they were rebuilt into steel cars (first M-15 conversion
was July 1934). See RP Cyc Volume 9 for more information.