I simply don't believe a formation of 24 Shacks could operate with only one engine feathered. By the time the last aircraft joined the formation the first two would have shut down at least one each, and in the third aircraft the engineer would be having his customary chat about oil leaks with the beam lookouts.
Blacksheep, a very observant comment, you've almost hit the nail on the head. It was two engines stopped but not feathered thus giving GL12 to GL24 time to join the formation. The engineer to lookouts "chat" was actually more of a "broadcast".
Happy to have clarified this point but please try not to force any further classified material to be published.