Moonliner ,
I have quite a few variations of this type of cabinet. Make reference to my post # 2831 in this thread . The largest in my collection measures in @ 30 X 20 x 25 1/2 and I was told by the fellow that I purchased it from that it was a remnant from the Wall of Sound . I also have several that I have had since about 1975 that I believe to be Alembic A-15 cabinets, those measure in @ 26 1/2 X 17 X 21 1/2 . I also have an early Bagend that is smaller then the Alembic A15.. SO you see there were a few folks making these with a Infinite Baffle design , Birch plywood and the swell aluminum brackets ( I have also seen some variations with tuned ports ). I have had discussions with friends that knew some of these speaker cabinet builders and it it sad to say that not all of them are with us anymore here in this dimension .
Perhaps, It might be accurate to speculate that dimensions may have varied just a little from batch run to batch run depending on who was calling the shots . Having been involved in similar activities this has been my experience.
It would be prudent to find out the exact model number of the 15 JBL Driver that is in the cabinet that tcmted ( the seller) has in this cabinet . Also what the impedance is. That all makes a big difference . I know of the variations that were used and how well they held out and through my own experiments as well with these.. I believe towards the last part of the existence of the 'Wall of Sound Gauss drivers were used ; Gauss 4580's work really well with these. I prefer to use the Gauss 4580's in mine for playing Bass. I have 2 Alembic A15's in my studio PA for lows that still have JBL K140's , that a friend of mine recently re-coned for me. Earlier incarnations of 15 JBLS sound good but in this application (Wall of Sound) but were not always as robust in lifespan . None of mine are for sale.