"That manifold is a modified B18 manifold. In its original form, it would have had the intake manifold attached such that exhaust and intake were a single piece. I think it's a 1967 piece - later manifolds were also combined intake/exhaust, but there were additional butterflies in the exhaust which hampered performance.

Unfortunately, the 1967 manifold is both the most rare and - according to many enthusiasts - the best original piece to use with SU carbs."

I thought about those in manifold throttles, they are the same size as what's in the SU's & it did meet emission standards. I always wondered would the butterflies closed improve fuel economy? Cold air & fuel gives more power, but, cruising down the highway with very little throttle a gasoline engine would like to be fed warm, vaporised fuel, that's why most every road car has the injectors aimed at the hot intake valves. Outright performance needs injectors mounted further back in the runner so the fuel goes in atomized.

Rarity: It's the opposite way around here. I only have one of the manifolds with emission throttles. It came in the 1st of the B20 142's here & the throttle shafts were flogged out in car with not much more than 100,000 miles. It got swapped out asap along with the emmision needles in the SU's.