The tube in the back is the blocking wires, I got medium sock blockers, & harmony circulars in size #4, #6, & #8 with 16" cord (they're so cute). I went with the 16" because I like the length for hats, and the 16" isn't included in the interchangeable set.
I particularly like these ones because the needles' shaft is short enough that it doesn't mess with your gauge as much. It's one of the failings of the Denise set, it has cords to make 17" & 20" circulars but due to the shaft length they are awkward to actually use. Also unlike most bamboo needles they have a pretty good point on them that doesn't split yarn as badly, and are more durable.
Sock blockers in action!