642 has a stainless cylinder/yoke, while the 442's cylinder/yoke is blued. 642 frame is clear-coated aluminum alloy. They have had issues in the past with that clear coat peeling off at wear points, which then causes the alloy to begin degrading if the gun is carried/used a lot. All of this was some years back, however… haven't heard of any recurrences. The 442's alloy frame is black-anodized.
.