The word secondary in the phrase 'Mechanical secondary' refers to the secondary throttle blades. Carter & Edel 4bbl, QJ & TQ are all mech sec carbs, as are some Holley carbs because the sec blades go to WOT when the primary blades go to WOT, connected by a mech linkage.
A Holley vac sec carb is not a mech sec carb because it's sec blades are not opened by a linkage from the pri shaft.
