Out of curiosity at my last Waveguide shoot out, we took apart a BMS compression driver that was featured. Above you can see the magnet and below the diaphragm.
This is described by BMS as an annular diaphragm. You can see the voice coil and it's former. The diaphragm is the clear plastic ring to which the voice coil is attached. Beneath it you can just make out some radial slots. The sound enters these which form the beginning of the phase plug. The sound waves move downwards in this picture, which is actually towards the rear of the compression driver. They then come out the middle inside the black tube, so the sound waves have travelled through a 180 degree rotation. This is similar but different to the Faital compression driver (HF10TX) in which the sound waves move forward the entire time. The exit tube appears to have a straight tube, followed by a conical section. The transition is not as smooth as it could be, but overall the exit tube is more smooth than the B&C DE250.