This would have been between 1977 and '88, according to the auction blurb. After that, Ludwig swtiched to Duntech. FInally. he switched to EgglestonWorks Ivys, encased in granite.
It doesn't baffle me why there's a protrusion of carpeted wall standing between the speaker arrays. It was suggested to me by an acoustician who visited the studio years ago that putting a wall-like barrier that runs perpendicular to the front wall, right in the middle of the room, to some degree towards the listening position, would help improve the channel separation of the speaker response.