Guy Wilber wrote:

in 1914 The Master Car Builders Association adopted, as recommended practice, both the size and location of placard boards on the sides and ends of house cars, The placard boards were to be no less than 16" x 24" in size, The end and side location was to be no less than 4' 6" to the bottom of the placard board from the floor line. Placard boards were to be constructed and firmly attached to steel doors and ends, wood cars were to have the designated area painted with a contrasting color (preferably black) if separate boards were not applied.

In 1953 the AAR's Car Construction Committee lowered the location of placard boards on all the (then) current plates covering doors and ends of house cars. The new height to the bottom of placard boards on sides of cars was 22" and on ends was 2' 6". While The Bureau of Explosives agreed with the changes, the new lower locations were not officially adopted until 1954 after all Interchange and Bureau of Explosive regulations were submitted and approved by the membership of the AAR. Despite the delay in official approval many cars were built and rebuilt with placard boards located in the lower position. Ed Hawkins and others have noted this from photo evidence during previous discussions.

As always, a good summary from Guy. The AAR Manual page covering this change in placard location is shown in one of my blog posts, link below; that post was part of a series of posts about route card boards. In each post, the prior one(s) can be found by links to the earlier information. It will be noted in the AAR Manual page, incidentally, that the change was Recommended Practice, not mandatory.

http://modelingthesp.blogspot.com/2012/01/route-cards-7.html

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