Frankenmags were Bushmasters attempt at a legal, 10 round magazine.
They took postban 20s or 30s, cut a section out and placed an aluminum block in there (seam line in lower 1/3 of mag). Rivet the thing back together, and hope it works.
The problem is that the BATF required such a tight spring and close tolerances to ensure you couldn't cram an 11th round in there, it was tough to get the mag to work correctly. Also, you can't dissassemble the mag for cleaning and maintenance.
I don't blame Bushmaster, blame Klinton, Brady and the BATFags.