Hi John,
I saw this reference on a shop drawing for part of an aircraft hangar built during the war in the Florida Keys. I was doing the architecture for its reconstruction.
We had what we thought was a complete library of MILSPECs but couldn't find the referenced specification.
So I asked.
It means "Make It Like The (fill in your own obscenity) Drawing For Once."
I usually saw (F) or (GD) in the parens.
Your work is astonishing as is the design of the parts.
You say this is the work of engineers. I wonder if they had brothers who were machinists who needed plenty to do. As Kolomann has suggested, these must certainly have been very complex aircraft from a construction viewpoint.
Thank you for sharing your work.
John