John Hilliard's system is also featured in Alexis Badmaieff / Don Davis' "How To Build Speaker Enclosures" (Howard W. Sams & Co., Inc., 1966), ISBN 0-672-20520-3, in Chapter 3, Infinite Baffles.

Here's an archive.org link to the 14th edition; it's described on pages 37-40.

How To Build Speaker Enclosures By Alexix Badmaieff & Don Davis : Free Download, Borrow, and Streaming : Internet Archive

Fwiw, Badmaieff was chief transducer engineer at Altec and we all know who Don Davis is (SynAudCon, etc). They also co-authored the initial edition of "Sound System Engineering".

-bob