Jake,
The forestay on the SC20 is one foot closer to the mast than on the ARC22 rig plan. Therefore to make a 22 jib work on a 20TR the foot must be shortened one foot and the leech must receive a big undercut, about 8 inches, to clear the mast diamond spreader.
Jake, just to have the history right: The 20TR came out in 1981 and the ARC22 in 1992. The ARC22 uses the same mast and mainsail as the 20TR. The jib on the 20TR is skinnier by one foot because the forestay is one foot closer to the mast than on the 22.
Bill