just food for thought......My FN FAL PS90's 10rd mags are just 50rd mags without the load indicator line over 10 rds.
The only difference between the 10rds and the 30s and 50s is a rectangular plastic filler block.
These are factory mags and without a doubt could be converted back. There aren't screws for disassemble, i believe they are "permanently" glued together.I would argue they are less easily converted back than any magazine utilizing a removable floor plate
Most mods i have seen to be compliant with 10rd max, have a simple rivet installed to limit the travel of the follower.
I wouldn't want to be the test case, I just use 10rd mags or true prebans. The trouble is there aren't preban mags for all firearms.
*fun fact* ps90's and fs2000's are two of the few guns that would require removal of parts rather than addition of part to become full auto.