Gun makers aren't normally in any particular hurry to identify their vendors. Those vendors could change at any time, anyway, and there may be more than one vendor for any specific part or assembly, as well.
I've seldom heard an armorer ask about what vendor makes what parts in any of the armorer classes I've attended over the years, and the few times someone has asked, it's been a coin toss whether the instructor or factory rep had that info himself.
How about this, though ....
Over time, having attended the M&P pistol armorer class 5 times (and the Shield class once, so far), and occasionally calling back to order spare parts, the subject has come up. Once I was told the manufacturer for S&W M&P pistol magazines and springs was Italian. Another time I was told that the company had changed the vendor for their M&P 45 magazines to an American manufacturer. I didn't even bother to ask when I did my last M&P pistol recert, and then a Shield armorer cert.
In the past, during the course of attending the S&W 3rd gen pistol armorer class a few times over the years, I was told that the company had used a couple of different American manufacturers for it's magazines, as well as its magazine springs, but the factory people who told me that didn't know the names of the companies, or which was making which magazines for which models of their 3rd gen pistols (or springs) at any given time.
Another time I got the impression that the mags for one new model were being made in-house, at least part of the time, but I didn't specifically ask for clarification be3cause I was calling on a different, although related, matter (as an armorer).
When it came to the SW1911's it was a bit easier, as the factory used some different products from some different well known companies at various time. The impression I got when asking about the changing selection seemed to be the availability from any given company at the time. I was told at one time that they had 1911 mags from a couple of different companies being shipped with their guns.
A couple of the oldest and/or largest pistol magazine makers are in Italy, and last time I remember some passing comment about magazines for a German pistol, it was mentioned that the Italian company making their mags made mags for a lot of gun companies.
In my SIG (Classic) pistol armorer class several years ago it was mentioned that for a couple of the models there were magazines made in both Italy and Germany. The only reason we were told that was to explain the differences in construction and appearance, and to remind us not to mix up the followers and springs between the different manufacture mags (from the different manufacturers in the 2 countries).
I completely understand that many private owners are gun enthusiasts and hobbyists, and like to learn interesting details about their favorite guns. I've met my fair share of armorers throughout the 26 different armorer classes I've attended (I actually went back and counted the certificates last year), though, and most of them are working folks who are more interested in being able to support, maintain and repair the guns for which they're responsible, and not so much in learning all the myriad details about manufacture and vendor sourcing. (Kind of like most of us weren't interested in where the tires were made for the patrol veh's, or the components for the radios and computers, etc.

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Another sometimes interesting twist to this is that sometimes gun companies might not only change vendors for various aprts and assemblies from time to time, or use more than one vendor at the
same time, but they might actually end up buying a vendor.
S&W did that with their longtime major plastic/polymer vendor a few years ago. Then, they decided to buy the equipment to start doing their own nitriding process (and do it for other gun companies, under different names, which may also involve signing NDA's, so the other gun companies can call the treatment whatever they wish since they're "buying" it).
Don't expect any particular customer service person to have the current knowledge about what vendor is making the magazines, either (or any other parts), as they often only know enough to be able to do they jobs. One CS person, might, and another dozen might not. One CS person might be a 30 year S&W employee who has worked in production for some different guns, and enjoys answering phone calls ... and another one might be someone who just came over from their retail store, and who has never even fired a gun, but can tell you all about apparel and other products and services offered by the various companies S&W's holding group has been busy buying in recent years.