The only differece I can see is that they lengthend the forends on their mil and LE shotguns,and the hunting ones still use the shorter ones ,
if you were just repacing the forend,the wood or plastic furniture,then you'd need to know which one to get, but since you're replaceing the whole slide tube assy,you should be able to use ether.
One more thing are you going for wood or plastic? I think the wood only comes in 6 3/4",but not 100% sure.