My guess on the cutoff is that the receiver sight is attached by something that replaces it. Perhaps like this one -

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Maybe the rear swivel mortise on a cutdown rifle stock has been filled? Maybe a custom "Mannlicher" style re-stock job with no swivels? Lacking photos, we're dancing in the dark here.

esdepew, this is an image of a carbine "altered for knife bayonet" with what collectors would call a "full-length" stock (also with swivels, handguard, issue sights, and an upper band).

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