Are you a lefty? I'm not, but the sling always gets in my line of sight too. I switched to an ambi sling, and replaced the rear push-pin with an ambi pin. It mounts the sling just a few inches lower in the rear, just enough to keep the damn sling out of my face.
Both sides of the stock are identical. It did not come with rear sling attachments with the exception of the loop located on the shoulder portion of the stock itself.
I got in contact with Adam Weber from hkparts.net and he explained to me that this was a rare hybrid stock that HK put out. It was a precurser to what we know now as the "F" style stock. Apparently HK produces a limited number of experimental stocks to get user feedback. Once they get the proper info HK then mass produces the product.
I use a factory MP5 sling on my gun. What I did was put a small loop of 550 cord through the loop and tied it off with a square knot and burned the ends. It doesn't look pretty but it gets the job done. I find that having the sling attached directly to the rear of the stock keeps the stock out of my face during transition.