501 filter cover used to hold Particulate Filters 5N11 for half / full face gas mask 501 Filter Retainer. Half Face Reusable Respirators offer the user comfort and convenience coupled with the cost savings of a reusable respirator. Reusable - just dispose of the cartridge and filters when used to capacity. The swept back design of the cartridges improves balance and visibility. Can be disassembled, cleaned and reused. Very light-weight. Easy to adjust head straps. approved N95. Case of 100. The Filter Retainer 501 is used to hold Particulate Filters 5N11 and 5P71 in place on top of 5000, 6000 and 7000 series a respirators when used with a respirator cartridge.