Robert, The part number is for the front heater core which is 20% larger than the rear one I use, plus it has 3/4″ tubes instead of 5/8″. I have ordered the Spectra part number, and about 1 out of 5 times I get the one pictured. The other times one of the funky brass, copper, and aluminum one shows up. It’s crazy, but they have the same part number. Autozone shows a Pro-Source OEM style replacement rear heater core that is all aluminum, and the one time I ordered it, the correct one showed up!
You are correct, the fin count per inch is one thing that make these work. They also have “flat oval” tubes, and the wall thickness of every piece is super thin. At AMP this year, Allen and I both had to put two strips of tape over the nose opening to get up to 140deg, and it was in the 70’s, and lower 80’s. Many times at MIS I ran one strip of tape on the nose to get to temp.