The Trieste J had a built in reserve mechanism.  Many divers liked this option so they did not have to have J valve tank vales on their tanks as the were more prone to leak.  The Trieste is described by some divers as the the best that Voit ever made.  I think this is greatly inaccurate.  While it is an elegant regulator and very unique I do not find it comparable to a US Divers Royal or Mistral.  I think that the small diaphragm casues a mechanical disadvantage as compared to other vintage double hose regulators.  I have dived many Triestes and do like them however.  One very nice feature of the Trieste regulators is the low and high pressure ports.

Action shots of the Trieste