More that likely they were just using up some old frames they had in the warehouse. After all S&W does have a rather long history of releasing serial numbers out of sequence.

BTW, for those blaming the 2 piece barrels for this failure, look closely, that's a heavy profile one piece barrel that came apart. If I'm not mistaken the 2 piece barrels used the same half lug shroud as the 620 that covered the ejector rod and keyed into the frame. They also failed in an area which is typical for a failure of the one piece barrels, normal mode of failure for the 2 piece barrels is at the cap on the muzzle end.

Possible causes that may be at work. One is that some assembler got really stupid when tightening the barrel down. However I would expect a failure due to overtorquing to cause a failure at the transition between the extension and the barrel body, not within the threads. Second is that the mating face of the frame wasn't perpendicular to the bore for the barrel extension. This could cause a failure such as seen here. Basically tightening the barrel would impose a bending moment on the extension and depending on the strain path it could cause a failure at the point shown. Personally, that's what I suspect to be the cause in this case, it's a rather odd location for that failure. Finally it could be due to bad metalurgy, either improper grain structure or an alloy that didn't meet the specification.

If this failure is even a bit widespread I expect we will be seeing another recall. Especially if the cause is due to poor metalurgy. If S&W follows normal industry practice, they rely on their supplier providing material that meets specification and is certified to that specification. This means they don't have to pay to equip and maintain a metalurgy lab, they pay a bit extra to have the supplier carry that cost. It also means that the supplier will have to pay for the full cost of any recall that can be attributed to material that doesn't meet the specifications. BTW, you can bet that these barrels will be subjected to lots of testing at an independent lab, a quality manufacturer like S&W gets VERY VERY upset when a supplier screws up like this. I also suspect those frames will be gone over with a fine tooth comb and that things will get VERY tense in the QC areas.