No, it won't work.
There is one of these side swivels for the skinny barrel 6520.
they are the same as the ones for the 6920's except for the cut out for the barrel.
It is NOT the double sided one, where you just flip the block to fit skinny barrel and M4 barrel.
It is flat on the outboard portion of the block (just like on the current 6920 swivels).
The top and bottom edge of the "U" on the swivel portion is thicker to fill the extra space between the FSB and skinny barrel.
It is made for the pencil/skinny barrels ONLY and is stamped with a different part number.
The construction is identical, only the dimensions and part numbers are different. I have had both in my hand at one time. they do exist but i have no clue where to get one, i 'd like to find one as well for my R6520.
i can't type tonight, sorry.