JM - When you buy MM enamels from the rack, look for both the guide that is part of the display rack, AND the bottles individually, for FS numbering code. The numbers 1, 2 and 3 will indicate gloss, semi gloss or flat. That will be identified by the FIRST number in the FS code, both on the rack guide and the bottle.
Example as read from my stash - MM Gloss Gull Gray is FS 16440
MM Flat Olive Drab is FS 34087
As to drying in bottle after opening, I always clean the threads, top rim and cap interior thoroughly before storing. A messy top area allows air in while stored. A quick wipe with clean tissue and thinner cleans it all like new. I have bottles that are years old, still very useable.
I never shake the bottles to mix, I use a metal dental spatula for complete stirring, then a Badger paint mixer for final blend or adding thinner. The bottle top just stays that much more clean and the paint remains usable much longer. I avoid pouring out of the bottle, I use straws, finger over one end, pull out whatever paint I need, neat transfer that way. Hope this helps.
Patrick