I don't know.
However, Santa Fe used "L" (for "leading"?) as the designation of one of the A-units of a multiple-unit carbody locomotive set: an ABBA set of F units would be designated as units L, A, B, and C, with C being the other A-unit. Their PA locomotives were delivered as three-unit ABA sets. So, for example, the first set, I think, would originally have been numbered 51L, 51A (the booster), and 51B. (Or originally planned to be so numbered: the possibility of having different trains headed by A-units of the same number seems to have been undesirable enough that they were renumbered, perhaps before delivery, as 51L, 51A and 52L. I think.)
Anyway... There was a set one of whose units -- originally, I think, the 60B -- got renumbered 77L. I think. (Sorry, I have just looked at the roster at thedieselshop.us, which isn't as clearly laid out as it could be.) It is at least possible that the "L" painted on the front marked that this was the L units of a set.
Real Santa Fe experts please correct me.