Yeah, Altec offered some duplex cabs with a shelf vent variant which I initially used in my 20 ft^3 dual 515B 'subs', but with the advent of real LF capability I tuned it much lower, first with a longer shelf that extended up the back wall, and later with a T/S vent to get around the strong TL action's upper harmonics mucking up the lower mids when I began using them with a higher XO point.

Some folks believe the Onken is mildly loaded this way, but it's not, though its vent system does have some resistive damping due to the large summed surface area Vs one or two large round vents of equivalent CSA.

GM