Those would be considered 20” firm joint calipers. That size was not super common. You might get some clues in looking at the shape of the joint fastener.
I saw your L as a mis stamped D.
Lufkin was good at putting their full name. Your L could be a user iD.
In my Firm joints, I have a B&S, 2 Starrett, 2 Union, 1 Coastbilt, 1 Hessenbuch, 1 unmarked, 1 Meigs Powell. I will give a glance at the joints later, see if any are close.