I believe the blackface Concert Bass is the same as the earlier silverface Concert Bass. That amp was described as being able to handle two cabs, although I see in my '72 catalog they show it with one 610S, one 312S, TWO 410S, one 412S and TWO 212S.
The '74 catalog "mix and match" chart suggests the following cabs for the Concert Lead: one or two 410S, one or two 312S, one 412S, one or two 610S, one or two 115S, one 215S, or one 612S. In 1975, Sunn suggested one 612S, two 410S, one 312S, two 412S or one 610S.
That said, here's a really cool clip from Soul Train in 1973:
http://www.youtube.com/watch?v=WM5gLyKBGSU&feature=related. In addition to Mr. Dynamite, James Brown, you'll see his rhythm guitarist has a Concert Lead with two 412 cabs. (The lead guitarist and bass player have Coliseum rigs.) As JB would say, "Make it funky now!"
I may have made this less clear rather than more clear, but I figure by '75 Sunn had a pretty good handle on which cabs worked best with the Concert Lead.