As it was explained to me a while back, the first "new" production run of the 642-1's occurred when the company wanted to clear out some old-stock frames. I think one of the guys said that maybe 3-4K of those older frames were taking up space in their parts vaults.
They pulled the frames and used them to make new guns, which meant they were given new serial numbers as they were pulled and used, reflecting the current serial number range. I was told they also used up some older stock of 442-1 frames the same way.
The limited run of those guns were spread among some of the usual distributors they used, and they sold out FAST. I got the distinct impression that it caught at least some of the people at the company by surprise. One guy with whom I spoke didn't even realize they'd made that short run of 642-1's until they'd been made and were gone.
I'd owned a late-production 642-1 (marked rated for +P) for some years, and I liked it so well that I thought I'd pick up one of the new ones. I called to get an armorer price on the new 642-1 and was told they'd sold all of them, but they gave me the names of a couple of the distributors who had received the last shipments. I gave the names to someone I knew at S&W LE distributor and asked him to try and find one. He did, and I paid what I felt was a bit of a high price.
Oh well, supply & demand.
S&W finally decided to go ahead and start making new runs (new production "-1" frames, meaning sans the ILS) of some of their more popular Centennial model J's, and they've remained a cataloged item (although still having their own production schedules).
I also picked up a second M&P 340 once they introduced the No-Lock variation in that model. The first M&P 340 I ordered was sold from the first production run, and it had the ILS (lock). After inspecting it to make sure torque lock's little dog leg spring (of the locking arm, or "flag") was correctly installed, I've tried to beat that J into submission with Magnum, standard pressure and +P loads since I got it ('05?).
I still carry & use that first "lock" M&P 340 more than my No-Lock M&P 340. It's also been my "practice" revolver for keeping my armorer skills from rusting away. I've think I've cut 2 or 3 extractors for it, using the hand-cutting tool S&W makes & sells to armorers. (Not exactly a recommended practice, but who else is going to let me practice my skills on their guns?
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Congrats on the new 642-1. there's a reason it's the best & fastest selling J-frame the company makes.
The only additions I've done to my 642-1's is to paint the front sight's ramp to make it easier to see and pick up.