Yes, those are the old-style "frankenmags", made by cutting the curved portion out of a post-ban 30-rounder and pinning the straight portions back together on a block of aluminum. They are generally a crappy design and should be avoided.
The new-style 10-rounders are better (plastic, non-removable floorplate type).
Still, if it's legal for you to do so, I recommend sticking with USGI 20s and 30s.
-Troy