Erratum: (et al.)
I misidentified the BGT sling; it is indeed a M-1923 web sling, and is not wrong. I have corrected the text of my tutorial to reflect this.
Also, for those of you who own the BBi M9A1 Bazooka from Lefty McGill, you already own a mass produced version of this sling, complete with "S" hook:
The BBi M9A1 Bazooka:
Close Ups of the sling:
Thank you to Santos Airlann Balaton for brining both of these facts to my attention on Facebook.