the non kerfed box is strictly 1/4 deteremind by the speakers Fs the kerfed t-line the compression chamber is the one along the back but you can't notice it because of the camera angle.
The kerfed t-line isn't like an ordinary t-line, the calculations are completely different of a regular t-line and to me it works so much more efficiently. first it goes loading chamber, expansion, compression, then acceleration. each chamber varies slightly in area.
but yeah the second t-line you see there is strictly 1/4 wave based on the speakers fs of a mtx 12 inch jackhammer and i used the Sd to determine the height and width.
also i forgot to mention on the kerfed t-line the port area is determined off of the actual cone area of a 12 inch speaker and not the sd of that Fi sub.