I don't know for sure where this falls in place but my copy of All ABout
Tank Cars 1921 published by Standard Tank Car has a drawing entitled "Figure
6 Markings. (Revised)" on Page 579, which reads.
"1918: Addition of horizontal bars above and below initials and number."
"1919 "A.R.A. Spec III (or I, II, IV or V)" substituted for "M.C.B.
Construction" on new and restenciled cars."
Thomas N. Birkett, PE