Then the answer is that for those magazines you would get to discuss the intricacies of the law with a rabid prosecutor in front of a Judge and jury.
If you have something like the magazine from the below link and it isn't currently loaded with 9mm, you will very likely beat the wrap. But if a cop with a hair across his ass decides to trump up a charge, your mileage may vary.
If you are looking for a magazine for your CZ 75B, 85B, SP-01, Shadow, Shadow 2 pistol, choose the magazine manufacturer that is the most trusted in the industry.
mec-gar.com
If you have a CZ in 40 that magazine is clearly marked as 40 cal and limited to 10 rounds of the correct ammo.
If you're found with that same mag in a 9mm CZ then you are going to have issues trying to explain how it doesn't fall under the AWB.
For an AR platform magazine, you are going to have to overcome a higher hurdle
AR Style Magazine, .458 SOCOM, 7 Round, Polymer | Lancer L5 AWM
shopwilsoncombat.com
Take a look at the mage body - it is clearly marked at the 20 round mark.
Will a judge allow into evidence the mods the company makes to allow better feeding of .375 SOCOM?
Probably but he will also allow evidence showing the magazine will also feed 223/5.56 even if it sucks at it.