Last I heard, the 642 was still pretty much the most popular selling revolver being made by S&W.
Heavy demand probably makes their production runs go quickly.
Then, you might have to consider whatever rotation schedule they use for production of the various models, and how it might limit availability of any particular model at times throughout the year.
I remember when I wanted to pick up one of the M&P 340's just being made without the ILS (to complement my existing M&P 340 with the ILS, made from their first production run & release). When I called to get an armorer price, the fellow on the phone had me wait while he checked to see if a recent production run of them had been done, or how long it might be until another one was scheduled. Fortunately for me, they has some in-stock that hadn't yet been shipped to a distributor.
Before that, when I wanted to pick up a second 642-1 (since I liked my original production 642-1 so much
), I also called S&W to get a price. That time, however, I was told that they had none left in their vaults, having sold all of the first few thousand of them they'd made in that first "new" production run of 642-1's (made from old frame stock, gathering dust in their parts vault). They gave me the names of a couple of the distributors to whom they'd sold most of those 642-1's, and I had a S&W LE distributor call around and finally locate one for me.
FWIW, without wanting to get too involved in a debate of the lock v. no lock Centennials (which has been debated ad nauseam in this and other internet forums), even though I own a pair of M&P 340's with & without the lock, the one I've been shooting the most ... and seem to carry the most ... is the M&P 340 made with the lock. Go figure.
It replaced my original, well-used 642-1 as my primary "range beater" J-frame, so it's seen a fair amount of use. I've also used it as a "practice" gun as a S&W revolver armorer.
As much as I like my 642-1 Airweights, I look at the M&P 340 as an "improved" 642. I liked the stronger Scandium aluminum frame, the blackened stainless steel cylinder (PVD coated) and the front XS night sight (with the generous U-shaped rear notch made for the round front night sight). The sight set is especially neat on the M&P 340. I presently own 7-frames, and that M&P's XS front sight, with its bright white plastic ring, is easily the fastest & most easily picked up front sight during rapid shot strings of aimed shots. Especially for my aging eyes.
The fact that it's chambered in .357 Magnum was nice, but not the reason I bought it. If they'd have been making a .38 +P Only version at the time ... (which they later reportedly made at the request of LAPD, for optional use as an approved secondary/off-duty weapon, with the ILS/lock, BTW) ... I'd have bought the .38 Spl version.