But - thanks for bringing it up, since I had to research it to get the answer (my memory is not so great, either!).
IMHO it's a useful idea under certain conditions (obviously not racing-only), but wasn't really popular for the L6.
Just a guess: the D-P L6 manifolds were only for 12 port engines, since the effect is partially reduced when the primary-only runner meets the siamese opening and "and dumped into the common port" in the 250/292, etc.
