Regulators purchased prior to August of 2004 were all 1.5 liter per minute.

The Alco-Sensor FST and the Alco-Sensor VXL instruments require a 6 liter per minute flow rate to capture an automatic sample.  (Manual samples can still be taken with the 1.5 lpm barb if your local procedures allow.)


 
The barb unscrews from the side of the regulator.
 
Note the color of the barbs.  The 1.5 lpm barb is silver in color (left) while the 6 lpm barb is gold in color (right).
 
While they look very similar the difference is inside the barb.  Here you can see the diameter of the center hole that the vapor from your dry gas standard must travel though to get to your instrument for testing.

The left (silver) barb has an obvious smaller diameter hole and has the 1.5 liter per minute flow rate.  The right (gold) barb has a larger diameter hole to allow the 6 liters per minute flow rate.