Thom,
According to the registration the other SC20 had an oversize jib and a third set of trap wires and I guess it sailed with three people on board. The oversize jib had the jib tack on the spinnaker pole and sheeted to the old jib trim point about 3ft aft of the main beam on the tramp track. This makes a Genoa type jib something like the old TheMightyHobie18.
This boat received the same corrections for square top mainsail and spinnaker/gennaker as our SC20TR plus he picked up .981 for the genoa jib and .995 for the additional trap wire set. This gets you to 59.2 from 60.6.
Bill