Spare/replacement solenoid for the “#1 Stop” CO2 base plate throttle stop.
There are 2 different versions of this you can purchase.  Normally, customers purchase the unit that came with the throttle stop for ease of changing and swapping over.
There are 2 manufacturer versions available. “SMC” and a “FABCO” versions.  If you can not read the manufacturer on your solenoid, match up to the photo – the FABCO is the long gray coiled wire solenoid.