answer my own post
Did some searching. The "NOBUCKL" sling was issue for the P17 (14?) Enfield rifle for the US army in WWI. You know, the rifle originally mfg for the Brits in .303, then in '06 for our forces due to the lack of Springfields.
The NOBUCKL likely saw some use on Springfields as well. I'm sure they drifted into civilian use, post war.