Dust cap for the PR900W 4.5 / 5.5 reservoir. Fits over the reservoir terminal and covers both holes. Fits tightly and will need some breaking-in time before it will slip on and off easily. Please do not twist the cap whilst trying to pull it off, as that may unintentionally torque the reservoir end.
Feel free to lightly buff the cap inside prior to affixing it - the cap however, for maximum ingress protection, needs to fit reasonably tight.
Please also swab the hole out w/ a bottle brush thoroughly, prior to affixing the cap and repeat this after each fill. Keep the cap on whenever you are not filling your PR900W or doing rifle maintenance.