Great memory Glenn, I would be positive that is the case, that is why some of the identification numbers in the parts drawing are "greyed out", there were few iterations of L-Jet, the big metal damper was on the early version, then switched to the later plastic one.
If you have the metal one, it is normally bolted to a bracket on that side of the engine bay, possibly off the oil dip-stick bracket?
It is more likely to give an issue at high revs I would think as I believe this vacuum is used to advance the spark timing? More revs = more vacuum = more advance on the ignition IIRC.
In terms of the idle problem, have you got the wax-stat cold start thing-ummy bobby under the throttle body? These are very temperamental at the best of times and often cause issues with the idle.