Well I finally got an official S&W answer to my question, "My 642-2 is a non-lock model. Is this unusual or in error?"

The answer is:

"No, we make 642-2 with and without the lock, it depends on how it is ordered.

Regards, Paul Gazda
1-800-331-0852 Ext. 2905"

However, I've still yet to see another 642-2 without the lock.