Regulators are necessary equipment used to accurately deliver calibration gas from a cylinder to an instrument. Each regulator is designed to fit on a specific set of cylinders.
Most of the RKI regulators are designed for non refillable cylinders. There are two common fittings these regulators use to connect to the non refillable cylinders. One common fitting is a CGA 600 connection and fits on the 17 (no part number suffix) & 34 (-01 part number suffix) liter steel cylinders.
The second common fitting is the 5/8 x 18 UNF fitting which connects to the 29, 34, & 58 liter aluminum cylinders (-04 & -02 part number suffix) and the 58 & 103 liter steel cylinders (-05 & -03 part number suffix).
Make sure the regulator you purchase fits the cylinder you have.