I replied to this on the 1941-1941 list earlier and several minutes ago, but Yahoo may be having issues. Here’s my response. My apologies for any repeated messages.
I highly recommend reading chapter 7 of Bob Karig’s “Coal Cars” book for insight on prototype freight car lettering. There is a wealth of information there that can affect how we letter our models for specific years in the early decades of the 20th century.
As I understand the situation as presented in Karig’s book, the ARA lettering guides were recommended practices, not true standards. They did become requirements but not until years after introduction. After USRA control ended, a new set of guidelines were approved. Sometime in early 1925, a change was made to alter the weight data. It had just been marked as Wt. and the new practice was to make this Lt. Wt. for the light weight of the car.
Change was in the air at that time as the ARA approved new recommended practices for car lettering in the spring of 1926 that seemed to take effect later in the year. The main change in these new lettering recommendations can be seen in the data presentation under the reporting marks. Load Limit is an added line while the cubic capacity is moved to the right end of the car side. Here’s a PDF file that illustrates the 1920 and the 1926 updates. Ignore the 1927 on the file. I created this when I thought the practices took effect in January 1927 but it was just a few months earlier.
You can find builder images of new cars produced in 1925 and early 1926 with the earlier lettering style applied. And there are builder images of cars produced after mid-1926 that are wearing the updated lettering practices.
As per Karig, these 1926 guidelines did become required on all cars in interchange service on January 1, 1933. Even then, there were some roads that followed their own path. Use prototype photos to guide your work. I just completed a Westerfield D&RGW box car kit and the prototype data noted the road did not update the data presentation on their older cars until the late 1930s and early 1940s.
If you model the mid 1920s, there are many lettering nuances. Chapter 7 of the “Coal Cars” book offers the details needed as you make lettering decisions. While new guidelines were in place, it took years for railroads to update car lettering. Two million freight cars are not repainted and relettered overnight. Cars built in the 1920-1925 period may not have been repainted until the early 1930s so their original lettering would remain in place, with appropriate reweigh data updates. Here are some examples of older freight cars receiving some lettering upgrades as time marched onward.
El Paso, TX