I have 2 sets. They ONLY fit non-reinforced mechboxes (ie stock TM or JG boxes) and a few others. They will not fit CA, ICS, Systema, or Guarder boxes.
There is a more detailed explanation I wrote for filairsoft.com here:
http://www.filairsoft.com/forum/show...t=24873&page=3
And as Crunch said, they are excellent.