Ultra Linear Magnetic loudspeaker : a very advanced design for 1971 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 mobile front plate allows a three-position setting between acoustic suspension and bass-reflex modes, with an intermediary position.
Japan saw a few lower versions not exported.
The smaller SS-7300 did away with the horn and variable b-r port but retained the multi-amping feature ; a comment about them here - more than very positive. The smaller SS-7200 just had ULM drivers.
The SS-7600 was replaced by the somewhat different but identical SS-7660 (or ULM-66) in october 1973 but in Japan only.
Elsewhere, it was directly replaced by the SS-8150.
The ULM speakers are much more beautiful than what the generally poor iconography allows to see : beautiful hand-made workmanship and excellent finish befitted well these very advanced loudspeakers.
The design of the frame/grille bears its period but it is a pity nobody works like that anymore.
As all high-end Sony loudspeakers : give them a listen if you can - you'll be quite surprised.