BDH was not the first to claim that the Exclusive Brethren Assembly was the highest court in the land. BDH's claim is the most extreme, but J. H. Symington said it first.
You find it in the Ministry of JHS, Vol. 104 page 188, which is a transcript of a meeting held on 27th March 1982 in Westfield. He began the meeting by saying,
The BDH quotation came 21 years later in Vol. 16 (meeting in Sydney, Saturday 12 April 2003)
J. H. Symington claimed copyright of the first of these quotations, and the Bible and Gospel trust claimed copyright of the second.