My door did NOT have an L-77 lock set. My door has been changed by PO to a modern residential lock set. However the marks on the door skin are round and much larger than the current door lock. There also is no other holes for the L-77 to have been mounted to. In addition to which my door is a suicide and has the second latch.
This door did NOT have any of the standard lock set. Here is a pic of my lock set. Clicking the thumbnail is ill advised if on a land line as it is somewhere over 2meg.
It is very clear in the pics that the original lock set was round and the cover plate was also much larger than a standard residential deadbolt/lock set.
Without being able to find an original lock set I probably will end up putting in a stainless commercial round knob lock set that is as stubby as I can find. I'm certainly not interested in paying $200+ to maintain authenticity of the trailer's original state as I am completely modernizing the entire interior.