smallblock offenhouser intake
Wow, haven't seen an Offy Dual Port 360 in awhile.
I ran one of these on my '69 340 Swinger back in the very early '70s and was very pleased with it.
Rumblefish360 is 100% correct with what he said. The other part of the engineering was when the secondaries opened up you got a cooler shot of fuel because the secondary half of the intake runner was on top and, theoretically, cooler than the bottom primary half of the runner.
Good luck with your sale.