There are two types, see owner's manual link above.
The plastic ones that engage a dimple in the mag tube are meant to be foolproof. Apparently people were having trouble with the springy type and I can understand that. But had a guy show up at 4-H trap looking for help with an 870 that had the plastic plug retainer. Before I could get there another guy decided to help and took off the mag cap and Ziiiiing! We did find the pieces and showed the guy how to install the retainer properly.
