With JBL's great success in selling their pro monitors to the Japanese consumer market in the 1970's, both Altec and EV tried to get in on this bandwagon. In the late 70's and early 80's, they both developed specialty high-end speakers just for that market. EV's and Altec's specialty speakers were never intended to be sold in North America, and to my knowledge, never were. The EV's were actually designed by a Swiss office of that firm.
I can pretty much guarantee that this version of the Sentry 500 was part of EV's Japanese market initiative that produced the Patrician II in 1984 as described on
this page. They were apparently not that successful, and soon dropped by EV.