Type of Enclosure?

I'll try at this question although I've never built a bandpass box.  The following pic:  is this similar to what you have?

The center chamber is sealed and provides an equal amount of air compression to both drivers.  All the sound comes from the ports.  Going by the descriptions on the enclosure page you referred to and on Brian Steele's site, it looks to me like this is also a 4th order.

The two ports in this example can probably be thought of as one single port because their tuning would be identical. Both chambers, port sizes and the sealed section behind the drivers are identical to each side.  The ports' responses will be summed.  Unlike an isobaric, you still have the SPL increase of using two drivers.

If what I say is not correct or is only partly correct, someone who knows more about it will chime in on this.  Meanwhile, before you rebuild just ask yourself one thing:  "Am I happy with the way this sounds?"  If the answer is Yes, rebuild to the exact specs of the original.  You can improve on wood thickness or type of material without changing the tuning as long as all on the inside is the same as the original.