Product review (3 stars): Doesn't survive shipping very well.
I ordered the Broan 634M roof cap from National Trade Supply.  I don't know if the other vendors that sell it on Amazon ship this product inside another box but I can say for sure that NT Supply does not.  Packaged only in the manufacturers box, the roof cap arrived at my door bent and twisted.  I have uploaded photos above.

Otherwise, the Broan 634M appears well made and of sufficient strength and quality to last a long time.  This roof cap is sold under two different model numbers - 634 and 634M.  The "M" version has a 6" round collar stubbed out the bottom that is 1 and 7/16 inches long.  That doesn't leave much metal in the attic to grab on to, so plan on adding an extension prior to installation.

The non-"M" version has no collar.  The installer is required to run ductwork (either round or rectangular) out the roof and the Broan cap is secured over top.