This illustration shows the unblown and Blown Gas, E-85 configuration: Fuel goes in the rear, comes out of both check valves.

In this configuration, the pill resides in the rear check valve holder.

https://www.alkydigger.net/proddetail.php?prod=EN4023.32assy

This illustration shows  the Blown Alcohol, or Nitro:  Fuel goes in the back and out one check valve.

In this configuration, the pill resides  inside the barrel valve.

https://www.alkydigger.net/proddetail.php?prod=EN4023-17.5