I bought my first Model 642 in 1991, when they first came out. I shot the dickens out of it. It went everywhere with me, being my warm weather off-duty gun and my year-round back-up gun. I always felt it was the perfect 5 shot .38, corrosion resistant, light for carry yet enough weight to deal with Plus-P, S&W reliability and quality.
I got the same type of crack in the frame earlier this summer. It cracked sometime during a box of 50 at the range. I found the crack cleaning it the next day.
S&W sent me a shipping label. A few days later, they sent a letter confirming they had received it and gave me a work order number. About a week later a customer service rep called, said the gun wasn't fixable and found me a current production version in inventory. She confirmed my address. I got my replacement Model 642-1 about a week after that.
S&W did not use my old parts on the new gun. The new gun has a different serial number. They also sent the gun directly to my house.
I know that S&W sometimes sends the replacement guns to the owner's FFL of choice, instead of directly to the owner. I don't know why. I assume they send it to who or what entity shipped them the broken gun, or perhaps it varies as a matter of the individual's state law.
One way or another, S&W will do their best to make you whole.