@713j
I think I miss read your prior post and thought you were asking what type on magnesium not manufacturer.

I can tell you from my research that the most thoroughly tested OTC vitamins and supplements is Thorne. But very expensive. They have exceptional quality control.

Life extensions is also a company with a lot of credits are also do a lot of effort for quality control of their products. They too can be expensive.

So who do I use. A company called Puritan Pride.
It has been in business for over 50 years and I have been using since 1990s. They have a good reputation for quality control but offer from time to time specials like buy 1 get 2 free. Thus I wait until those special come around to buy it as my wife takes is also.

I have read that Natures Made is also a good brand and available OTC at at almost any pharmacy or grocery store.

But if price is not an issue I have heard the most positive feedback on quality control on Thorne. I think though it is only mail order. But I think I just saw in on Amazon so you can check.

I got 25% off buying directly from Thorne via mail order as I was a veteran and they offered a veteran discount. The 25% is quite good to lowering their price. I think they also give the same discount for emergency responders, etc.