The Pelican 1503 replacement O-Ring is designed specifically for the Pelican 1500 case. The O-ring gaskets provide the Pelican case with it's watertight seal to ensure they remain watertight when the lid is firmly closed. These O-Rings should be replaced once a year or when damaged.

What You Will Need.
You may need a few drops of glue, generally a heat gun glue will do the trick. This will help hold it in place when installing and prevent the O-ring from coming out. 
Do not use any acid based glues.

Installing the O-Ring
1. Give the O-Ring a slight stretch. Do not overdo it, it’s just to give a little extra play as you are installing it.
2. Place a couple of drops of glue in the area where the O-Ring sits.
3. Gently place the O-Ring into the recessed area and work around the case. It is a good idea to start from the center of the longest section, to allow the stretching to reform back into the case as you work around back to the start.