post18 is not a dual concentric of the "old" style.
post18 has a dome driver for the HF and the diameter is much smaller than the the bass/mid VC.
In the old style dual concentric, the VC of both the bass/mid and the HF are ~2" and the HF is a compression driver with the dome inverted as in all other compression drivers.
The Kingdom Royal post61 (very different from the Westminster Royal) has an HF VC that is bigger than the mid VC. This seems to be a 3rd variant of the dual concentric. I have not seen a cross section of this driver pair.