1) Every Head racket (at least from the players' racket "silos", as they are now called by Head on their website) has a code. The code designates the mould that was engineered by Head. Obviously, not every mould Head engineers develop will be brought to market. The "TGK/TGT" designates, in all likelihood, the factory in China where the racket was made. When Head still produced their rackets in Austria, the factory code was "PT". So if you have, let's say, a LiquidMetal Radical MP made in China, you can look at the hair pin and it will say something like "TGK167.1-A TT" - that's what's on one of mine, which I have here on my desk right now.
2) The flex - Head will have a specified flex, but due to production tolerances, this will not be achieved 100% of the time. Don't assume that all pro stock frames of a given mould are exactly the same, they also suffer from that. - In fact, I have had pro stock frames made for one player and customized by Head which differed with regard to weight and balance even though they were supposed to be the same. It's not easy to be 100% accurate, and neither does it matter as much as people here want to believe. Back to the flex: It will vary dependent on weight, kind of grip, amout of silicone/other substance(s) added to the hair pin etc. But overall, assume that the flex is intended to be the same. Edit: It's also possible that there's a variant of the mould with different intended flex. This is marked by the digit after the mould: 231.1, for instance, which is the MicroGel Radical MP, and 231.2 is the same racket but with a stiffer flex. This also applies to the string pattern. Again, 231 is the mould, and .3 stands for a variant with a 16x19 string pattern.
3) Quality is subjective. The quality of the frame is generally the same, but if you do an individual customization job, that may enhance the "quality" in your eyes. Plus, Head pro stock frames may be equipped with their "finest calfskin leather" grips, which you can't buy officially (but they are available for the Custom Made rackets!).
4) Paint: Head can use any kind of paint they like. If they decide Custom Made rackets are glossy, they're glossy, if they decide they're matte, they're matte. Nothing behind that, really.
Hope that helps.