My 442 has the lock, her 642 (with pink grips) does not. We've put about 250 rounds through each so far with no problems.
WRONG on a 642-2!!! at least mine and it has been back to S&W 3 times over cylinder issues.
Open cylinder.
Hold gun vertical.
Hit ejector rod with left hand.
MANUALLY REMOVE 2 or 3 empties with ends of cases still in chambers.