There was a recall on the 624 for having some incorrect stainless steel in the cylinders of some of them. There is some controversy on whether it is the steel or the chambers were cut for .44 Magnum. You can search here for the serial numbers affected. I don't seem to understand how to search as I had no luck. Also, try to insert a .44 Magnum in the chambers.
Anyway, don't panic! There were very few with the bad steel. Smith checked a good many guns and put a red C on the end label of the ones that passed. They did not mark the guns! I bought a 6 1/2 inch 624 and it was under the recall. I sent it back, and it checked ok, but Smith did not mark the gun. The bad deal is if it checks as a bad one, Smith will not return the gun and there are no replacement cylinders. There are folks on here that have had some in the recall range that never sent them back and have still been shooting them without issues.
If you call Smith, they will tell you if it is under the recall and send you a shipping label, if you want to take the chance.
Perhaps someone here can pull up the serial numbers affected to help out!
Oh, very nice gun and they shoot very well!