The "bolt-of-lightning" splice drawing shows four stepped and tapered, matched cuts in the ends of the spslsiced pieces. The multiple bearing surfaces, joined by transverse clamping bolts, have the appearance of a stylizrd lightning bolt.
This common repair method was rarely used in original construction. The two timbers of the butt joint do not touch but are held in tension by steel fish (splice) plates with through connectors on the transverse plane. The drawing shows a nut on the opposite side of the square-headed threaded bolt to hold the screw.