Dave;
Here is a 733 dated 3310 that does not have the vertical ribs. I *believe* I've seen late `67 dated ones that had the ribs, but I wouldn't say with 100% certainty at this point.
At least now we know that the rib apparently still wasn't present on
early `67 dated carbs.
As far as being `74 dated, AFAIK all Holley's after sometime around 1972 went to the 4-digit Julian dating system, so it would *appear* that someone's either fudged those #'s, or it's a pretty unique mis-stamp. (treading cautuiously with my statements here)
Just trying to do my job and keep someone from getting stuck with something that's not quite what they thought it was.
Eric