This Varitone pickup was factory installed in a later model Buescher 400 Tenor neck. This was after Selmer USA had bought out Buescher. The original owner purchased two necks for the horn, both underslung, one with the Varitone pickup.
Yes, that is Elkhorn, WI….not Elkhart, IN. Youtube: https://www.youtube.com/watch?v=7KS_F6B9YSg And here is the Tour Booklet: http://www.wisconsinhistory.org/turningpoints/search.asp?id=1211