Since this question crops up every so often, and I found this link, I’d thought I’d share it:
http://www.fulton-armory.com/sling.htm
Note that they also give some of the rationale for why such a complicated sling was in US use, too.
The canvas '36 sling that followed on (as a better use of strategic materials) also had application as a marksmanship aid (that’s why the lower clip slips on).
The M-1 carbine sling, however never was intended as a marksmanship dev-ce, only as a carrying strap.
Perversely, the M-1 carbine sling could be used as a marksmanship aid, but on the M-3 greasegun (in asimilar way the MP-5 strap can be rigged).
For those of you with an obsession to pendantic accuracy (we know who we are) my original 1907 mikes 0.120"; my reproduction 1907 mikes 0.1085"–call that 0.0035" or 0.088mm at 1/35.
Hope that helps.