I guess it's because the sewing machine repair man (who was fixing my touch and sew) told me to buy 601s and 602s because they didn't have plastic gears. My point was to buy machines that wouldn't have problems, or would be cheap to fix if they had a problem. The 538 surprised me. I mistakenly thought if the number was under 600, it would be indestructible. He did tell me to stay away from 400's because of timing problems. No one has corroborated that opinion so far, though.
bkay