I always thought that 55 was about the end of the line for the New Departure bicycle division.
The Twin Streak must have been their last hurrah!
Did Mattatuck continue the production of it also?
If you look at the 1959 Schwinn Catalogs, Mark IV pops up a lot in the rear hubs. Those were Sachs Torpedo hubs. Between Bendix and Sachs, it looks like ND got squeezed out from Schwinn which was probably their largest customer at that time.