The LD-340 to this day is regarded as an excellent high performance dual plane manifold- decades before the intro of the Performer RPM. But because of this the pricing has gone insane on the LD-340 units. Looks like the Port O Sonic single plane Offenhauser was their first array into legit scientifically proven single plane design for high RPM use. The design Offenhauser is known for is their split down the middle divided plenum single plane intake manifolds which seems to be the standard from the 50s and early early 1960s.