I'm wondering if that handguard was modified not by the people who prepared it for the Sub Target Gun Machine, but rather by someone who wanted to make it LOOK like it had.  Shouldn't a true STGM rifle have a hole drilled and tapped in the barrel for mounting?  It may be possible that someone had the stock and put it on a shootable action/barrel, and then modified a handguard to complete the look... not unlike someone who modifies a 1898 to make it look like a real 1898 carbine.  Now I'm not saying that is the case... but rather food for thought.

Either way, it is definitely an interesting find!