The third (wood) screw at the rear is a residual early feature and actually quite practical - keeps the sideplate from falling out when you remove both lock bolts. In addition to the undesirable loss of a sideplate, some of those early fowlers and tradeguns with serpent like and viney shapes, piercings etc, can get sort of bent and springy after being compressed by the lock bolts and could easily chip out the wood along their edges if they pop off off (personal experience
).
As Jim pointed out, some early English guns actually had a third bolt at the rear of the lock, which seems to have evolved to a residual feature in the form of the woodscrew on some fowlers and fusils (too keep the sideplate attached when removing the lock). The rear woodscrew crops up occasionally on some American longrifles, particularly southern guns, even some relatively late.
The tang looks to have been bent a bit to me - I don't think that is all wood shrinkage.
The rifle could stand on its own merits as but has the rare benefit of being known to have "been there" on the Kentucky frontier. If you are interested in the rifle, take the time to read the history of its original owner, Hancock Taylor, on Mel's website. One of the most astounding tales of exploration and adventure by an American in pre-Rev. times, and to think that this gun was there on the Kentucky frontier in 1774 and has survived places it in a category with few, if any other, known longrifles.
