From my own experience and just from wandering around the boards for a while.
I have both Gen 2 30 round and 20 round magazines, the 20 round magazines don't appear to be as popular as the 30rnd magazines, so perhaps that's why they're hard to find. In terms of reliability all of mine have functioned flawlessly with both brass and steel cased rounds from several manufactureres including PMC, Wolf and Tula. You really can't go wrong with PMAGS
There are some rifles that don't accept PMAG' such as the FS2000 and the FN SCAR16s (Note with the SCAR16s, it simply requires a dremel tool to modify the magazine so that it won't damage the rifle to make them ok to shoot in the rifle)
The latest Generation 3 "P MAGS' have a narrower profile so that they can be placed in chest rigs better among other minor differences. There are several pictures on the Cabela's website appears to carry the Gen 2 Pmags, you have to select which one you want. They appear to offer 2 color choices for the 20 rnd magazines, black and flat earth, while the 30 round magazines are offered only in black (The 30rnd magazine is shown in the third picture)
In regards to the numbers they appear to refer to the manufactures code, the aformentioned numbers specifically regarding Gen 2 PMAGS.
Each number refers to a specific model of magazine the acronyms specifying the color each model of magazine respectively
MAG210-blk/fde/odg: Gen2 Pmag with windows 30rnd
MAG211-blk/fde/odg: Gen2 Pmag without windows 30rnd
MAG217-blk/fde/odg: Gen2 Pmag 20rnd
Gen 3 PMAGS following the designations listed below respectively
MAG556-blk/fde/odg: Gen 3 Pmag with windows 30rnd
MAG557-blk/fde/odg:Gen 3 Pmag without windows 30rnd
I did not find a Gen 3 20 round magazine from Magpul Inc.
MAG243: 7.62x51/.308 20rnd magazines*
*NOTE: NOT compatible with Armalite AR-10B, Noveske N6, Springfield Armory M1A, Rock River Arms LAR8, FN SCAR 17S or any other rifles that utilize M14, FN FAL, or HK 91/G3 patterned magazines.*
If someone finds that I've made a mistake here, please feel free to correct me.
Hope this helped.