Har! I just got it. Boy am I slow. Only took a week!
You appear to have an (ahem!) earlier version personal skin than I'm using. I was issued M14A1. (Man, that bolt action lever would dig into my back.)
I will be chastised for the off topic diversion but I'm feeling particularly militant this morning. Perhaps this close to Veteran's Day I'll be forgiven.
Are you thinking of the M1903 Springfield? The M1 and M14 rifles both have an operating lever on the side to clear the chamber, same side as the M1903, so all of them dig into your back if slung the wrong way.
Actually, however, M1A1 Issue 1 more or less stands for the first production model of something, like the M1 Abrams tank (which will really dig into your back if you try to sling it...). Makes one wonder what happened to the M3-13 rifles? (M2 was the fully auto carbine, I believe).