I think that you've mixed up a few things.
Of course, that's no surprise; the NEC does not directly address the issue, despite there being some imcomplete references. The UL "White Book" does a better job.
Any securely attached metal box with a metal plaster ring - of any style - is good for supporting fixtures to 50 pounds.
The 6 pound limit you refer to is for 'old work' boxes, where the box is supported by the drywall, rather than by being attached to a framing member.
