differences are between Early (90-93NA,90-94TT) and Late (94+NA,95+TT) versions: Early lower plenum works only on 30P(NA) and 40P(TT) heads Late lower plenum works only on 10Y(NA) and 48P(TT) heads Both Early and Late upper plenums will work on Early or Late lower plenums (the ports are all the same size and alignment). External differences are: Late upper plenums have different bridging for the throttle linkage mounting, a large boss added to #5 runner, and the throttle linkage cover mounts you mentioned.
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