Reno:
Perhaps the interpertation at the time was that the pigtails satisfied the 6", as it allows the device to be removed & replaced. As I said earlier, I have been there with the old boxes, clipped corners, and also the pancake fixture boxes with multiple BX within. All were neatly arranged, some soldered & friction taped. The fixture canopies back then were IMHO large. Touch those neat conductors, and you were sorry!!
IMHO, it takes a talented electrician to touch/work on any of the 'old' wiring. Quite a few of the 'young' guys have never seen the things that us 'seasoned' sparkies have. We should be thankful that the 'old' ways (for some part) are no longer commonplace.
One of the courses at the vo-tech is 'Basic Wiring' and box fill, jacket length, free conductor length is taught. The instructor uses metal device boxes 3-1/2" deep, and NM. I am amazed at the difficulty some of the students have with these basic trade practices, and code compliance.