Ok here we go. Things to consider.
I don't think it is a typo or mistake. Also the reason why you don't see any other cars with this in photos is generally due to the oil staining on the domes. A reference to a flat car doesn't make sense to me. These cars were built as stantard gauge frameless cars.
LF could be from the standard gauge days. They all seam to have had at least 2 paint jobs in their life. The second being applied before conversion to narrow gauge. Gramps cars likely only had the gramps lettering applied on a dirty faded car. In photos you can see the AAR stenciling from out of state locations as well as tank testing in California. We added this to our Gramps car since we used the original number.
Our car had L only noted on the dome, and only on the right side, so nothing to do with left or right. It could have been on both sides and the other was just more faded away. We didn't add it to our car because there is debate weather the dome is from our car.(the dome was cut off before purchase from sundown and southern)
Bill and I have talked about what L or LF means and we have nothing. Hence the post.
It also has nothing to do with the type of heater. That is stenciled on the ends of the cars.
Placards were different back then. Here is a photo of possible placards used.
Jeff Taylor
CRRM curator of equipment and rolling stock.