Back in 75, the rhythm player in my band got himself a Concert Lead and a 610S. The blackface hadn't come out yet. It was a good combination.
At that time, Sunn had stopped bundling heads with particular speakers. Each was sold separately, and you could match the 610S with the concert lead or the Model T. Usually, though, the 610S and 612S went with the Concert Lead, while the 412S went with the Model T, either as a half or full stack.