The Mk. III mag was not intended to be taken apart. That is the reason you see so many that are "bu**ered up as someone tried.
If you are careful, you can sometimes take them apart by bending the tabs, but I have found most of the time, that a good cleaning with brake cleaner and then a spray of of LP 2 works just as well.
If you have a really bad mag and cannot fix let me know as I still have a huge pile of Ross parts.
I'll check and see if I have any spare bolt stops as they normally were not stripped out of the rifles when scrapped.