686-4
IIRC, the first revision built with newly-purchased CNC machinery. (Turns out Tomkins was good for something, I guess).
Don't remember if it was the first revision with the square extractor or not-- that may have been the 686-3.
686-5
First revision built with MIM parts and frame-mounted firing pin (as mentioned)
Round butt now standard* (Hogue conversion grips supplied on 4"+ models).
Hogue grips now have the ambi "thumbrests" and S&W logo
First revision with pinned-in front sight.
First revision with new style thumb piece.
First revision without the grooved backstrap**
First revision available with 7-round cylinder (as mentioned).
Not sure about transitional models (e.g. 686-4s with one or more of the 686-5 features). 686-4s were being sold in 1994 and 686-5s started being sold in 1998, so there's room for that sort of thing. Also not sure how the flash-chromed and case-hardened triggers and hammers were distributed across revisions. I'd hazard a guess, but I may have already overstepped myself by going too much on memory as it is.
There seems to be a lot of love for the 686-4, since that was the last one built without MIM or the lock, but built on the newer CNC equipment. Mine's got around 9k of my handloads put through it, and I'd consider it just about broken in.
In spite of what appears to be lots of cost-cutting measures, my 686-5 and the ones I've shot were all very well put together. The 686-4 (and others) built on the new CNC machinery during this time left a lot of people impressed (not *too* hard after some of the Lear-Siegler and Bangor Puta disasters, I guess) and was a hard act to follow.
Yep, there are a lot of nice, old revolvers out there. My grandfather's .32 Regulation Police is a prime example. (It's also in a tragic state of disrepair. See, those beautiful nickel and blue finishes require a lot of maintenance over the course of 70 years or so...) But, talk to some folks that actually carried those older revolvers for actual use and hand them a new S&W. You going to get a lot more "Damn, that's nice." responses that you may be ready for.
Some of us can truly appreciate the older models that were built by hand. We can also appreciate some of the advances that have been made and incorporated into the newer models. That "soul" that everyone is so keen on didn't come from the factory. It came, over time, from the owners and being used. My 681-2 already has soul. Given time, the even newer models will have plenty of it, too.
Still won't own one of those abominations with the damn lock, though.
* This may be a transitional feature. I don't remember.
** The 581/681s of any revision didn't have this, nor did they have the glare-reducing finish on the top of the barrel, IIRC.
