Roger
This is an after-market BT addition. Original BT forearms are not extremely common, and especially on lower grade guns; but on original factory BT irons, a 1/4 inch piece of metal was added to a standard iron and the joint has always been visible on every factory BT iron I've seen. This piece was added to receive the reinforcing bolt, which is also lacking. The factory made this modification to insure the heavier BT forearm did not become loose during recoil. Like you said, forearm checkering on this gun is not a Hunter Arms pattern; and from the pics, metal finishes do not appear original (lock plates appear to have been blued?). But a Trap Grade with a highly figured butt stock of American walnut is not unusual; I've owned one example and have seen others. Hopefully David will post some pics showing the details I've tried to describe.