Thanks Kevin!
I guesstimated by counting the fins times area/fin and comparing to other 5" extrusions on thermaflo site. But I didn't even see that one! Resistance is lower than I need so I already ordered a pair, and some other goodies too. Your site is a great resource of quality stuff at good prices. I am always tempted to buy more than I need. Cardas speaker posts, UcD modules, etc. All in time...
Thanks
Rich