Joe Gill joins Bridwell in scripting Secret Six 2 (June/July 1968), with art by Springer.
In this outing, Mockingbird orders the team to break into the Pentagon, using Lili De Neuve's make up skills and Carlo's magic tricks, and steal designs for a new bomber plane. They are then to sell fake plans for the plane to a foreign spy. Robbing the Pentagon proves to be the easiest part of the plan, partly because they have August Durant as an inside man helping them. But then they discover that the plans they stole are also a fake, and that real spies stole the real plans while the military were dealing with the Six.
So then the team has to track down the thieving spies, get the real plans and replace them with the fakes.
Along the way the team discovers that the spy who is behind the thefts is also the one who poisoned Durant years ago. So while on the surface this mission seems like the last one, in fact it proves to be specifically designed for Durant to exact his revenge. The Secret Six succeed, and the missile constructed with the fake plans blows up. The spy is executed by the government he works for for his failure.
The concluding panels of the story sure seem to imply that Durant is really Mockingbird. Apparently, Bridwell had decided which member of the team really was the one behind the group and, many years later, it would be confirmed that is really was August Durant. This means that not only was the Secret Six the first team to have a black member, it was the first team lead by a black man! Ground breaking indeed! And that also helps explain why, at the time, Durant would go to such lengths to conceal his identity from the rest of the group.