Ultra Linear Magnetic loudspeaker, with an all-new membrane and magnet / spider / coil / polepiece design to avoid leakage of magnetic flux :
a part of the polepiece is sanded so that the flux at the periphery remains saturated enough ; this reduces leakage to avoid negative feeback to influence the bobbin. The polepiece itself is then covered with a thin copper layer so that the non-linearity that happens at its extremities becomes shorted.

The plate allows a three-position setting between acoustic suspension and bass-reflex modes. Japan saw an intermediate model, SS-7400 - and probably a few others...

The smaller SS-7300 did away with the horn and variable b-r port but retained the multi-amping feature ; the smaller SS-7200 just had ULM drivers.

The ULM speakers were much more beautiful than what the german catalog these images come from allows to see - beautiful veneer and excellent finish befitted well these very advanced loudspeakers.