I would guess that they are referring to the AAR's Mechanical Division dropping the 1" bars (above and below) the reporting marks. If so, the change became effective on March 1, 1957.
This data is well documented. But there is an SP lettering drawing, with a note for the removal of the bars in late 1952, noted as “recommendation discontinued.” Both SP and PFE dropped the bars soon after. I confess I don’t know what that’s about.
