The PM4 frame is enclosed in a wood case.
The PM2 back wave passes through both sides of the horn. The horn load begin at the place marked in blue.
The balloon looking unit is for use in horn loaded enclosures to keep the high-frequencies from cancelling. It has the same function as the phase plug for the non-horn loaded designs, but is shaped differently due to the different high-frequency effects of the horn.
It is designed to make better use of short horn effect.
The PM4 tone is extended by the ball; also reflected by the short horn and the rear wall of room behind the enclosure. Finally, it is added with PM2 tone.