Off the top of my head, there were some occasional revisions to refine things that didn't get a lot of fanfare.
The angle of the cam loop (candy cane) on the trigger bar received a couple of angle changes with the sear changes (from 32 to 40 degrees?), but I'd have to dig out my notes. This was said to have been done to help keep the trigger pull within the normal spec for the regular series guns, meaning the Pro could have a lighter trigger from the get go.
The striker block spring was connected to the small spacer plate. This became a welcome improvement for those armorers who nick-named the spring spacer plate "The UFO" when losing control of it at the bench when reinstalling a rear sight base.
The extractor received a couple of changes over time. I remember one them as the hook being given a sharper edge and a more raked rearward angle. The reason offered was to improve extraction for some of the hotter ammunition seeing LE use.
Then, later on they made another change that was said to make the hook longer. The first ones I saw was in an armorer recert class, and those extractors had the letter "L" lightly inscribed on the inside (where it couldn't be seen outside the slide). A little later, when ordering a few spare extractors, I noticed they came without a hand-scribed letter L, but had been renamed to include an actual name in the part number (which I thought was interesting).
I noted that the part number for the trigger bar return spring was changed to reflect the same part number used in the .45's. (Except for the MA complaint guns, which I was told needed a heavier spring as one of the ways needed to bump up the trigger pull to meet MA requirements.)
The sear blocks were revised so the "standard" block became one which could be configured with and without the manual thumb safety option. The ILS (lock) capable sear block disappeared from standard production and became a special order.
The ejectors changed with some of the sear block revisions.
The slide stop assemblies were revised a few times, and not just for the ambi "pad" on the inside to be built up a couple of times, but to change the wire spring tension (color coded paint denoting changes and caliber designation).
The frame and sear block "rail" cuts were changed over time, and the newer sear blocks won't fit inside the older frames. This is one of the reasons armorers (or regular customers) might be asked for the specific serial number of a gun when calling the factory for parts, as the customer service person might need to look up the production vintage to be able to order the right assemblies for any particular M&P. BTDT.
There have been some further changes in some critical ways between the original (or 1.0 guns, as some folks like to call them online) and the 2.0 guns, other than the sear block/trigger interface, and some of those parts and assemblies are not interchangeable.
The RSA springs have been revised and tweaked a bit among the calibers.
In the almost 14 years since the M&P pistol was officially released (Jan '06) the company has been addressing customer feedback by making continual revisions and refinements here and there, but that's been S&W's normal practice for many years. In all the years I served as a S&W armorer for many of their guns I was always learning of some revision when calling to order parts. And that was just with their long running models like the 3rd gen pistols and their revolvers.
If I were to buy a used M&P from the first 4 years production, I'd replace the RSA, and probably the original striker assembly, slide stop assembly and perhaps the trigger bar (calling the factory to match the trigger bar cam loop angle to the production period of the gun and sear). The slide stop assembly and RSA are recommended for replacement every 5,000 rounds, anyway, but the striker assembly not for 10,000 rounds (9/.40 guns). The trigger bar for the 9/.40 guns is listed at recommended replacement at 10,000 rounds (but the trigger return spring every 5,000 rounds).
Congrats, BTW.
One of the things about the early 9's is that reportedly some of them might be tack drivers, and some "not". This was something we were told the engineers were trying to identify and address for the first several years. While the .40/.357 & .45's were usually described as being pretty accurate right out of the box, the 9's could be unpredictable and not as uniformly accurate. The ones produced in the last few years have demonstrated themselves to be tack drivers, though. I've handled and fired some later production 9mm "original" series guns (1.0's) and 2.0's, and they were very, very accurate.
Just some thoughts off the top of my head. No doubt I missed remembering some others.
