There were 400 PC guns made in '99 and 300 non-PC guns made in '03. The most obvious difference is that the PC guns had barrels with an angled underlug and the non-PC guns had full underlug barrels.
Assuming we're talking about the only 646 on gunbroker at the moment that gun does not have the original grips (they were Hogues) and doesn't come with the case, which was a small aluminum case similar to the PC cases. That being said, that gun looks like it hasn't been fired much, there's barely a hint of a turn ring and the finish on the titanium cyls shows scuffs and stuff pretty well. Might ask for better pics of the front of the cyl, it should have the same finish as the rest of it and it looks pretty smooth/shiny. If the finish has been taken off the front through cleaning the titanium will erode pretty quick just from firing.