I can't for the life of me figure out the proper way to route and fit a standard black web sling to a Colt R6520 with the old style two position stock.
Any help is appreciated

It goes thru the stock and over the top. Googlefu.

I mount my sling to the side going through the sling slot, other ways a sling could be mounted on the sling slot is upwards or downwards, users choice.

There is a buckle behind my stock which keeps the sling from sliding out.

My stock is the standard M4 type 2 current production stock but you probably have the older Colt Fiberlite CAR type 1 stock body, they're the same, they both have the horizontal sling slot at the middle of the body.

I'm a right hand shooter and I prefer to mount my rear sling at the right side of the buttstock, so when I shoot on my support left shoulder the sling doesn't strangle the front of my neck.

JOHN WAYNE SLING, BABY!

I'm having trouble finding a sling like that, can you point me in the right direction.

..or find a KAC QD adapter

Or an offering from Impact Weapons Components.
IWC Buttstock QD