Woops, the Grasshopper beat me to it, BUT:
i can't answer for sure on the 700/600 interchangability (i've heard they are though) but the 'spacer' is the recoil lug, and the M600 ones are set up that way to provide a recoil stop for optics mounts. a neat feature.
Last edited by toad; 04/27/11.
