John,
As seen in the D&RGW example, the year is important, as far as Amber vs. Green rear end markers.
It was not until the advent of the General Code of Operating Rules in the 1980's that classification markers were dropped from locomotives.
Rear end markers (red) are still mandatory.
The earliest rule book that I have that incorporates the Standard Code (frmulated at the same time as Standard Time) is a 1908 CB&Q book of Rules. I know that earlier rule books exist, I just don't happen to own one.
When I hired out, we were to display caboose markers at all times, day and night. by that time, the red daytime markers were reflectorized paddles that fit into the old class lamp slots. At night, we were to use the single marker displayed between the cupola windows.
In usual Railroad fashion "display of one marker is to be as restrictive as both displayed".
Rick Steele