This is an adapter for 3M half-mask and full-mask respirators with the standard "bayonet" style connector. This allows you to use the 5n11 style replacement filter media instead of the expensive and hard to find cartridge style. This is typically done with a 3M 603 adapter and 3M 501 filter retainer (which is compatible). Both are very hard to find currently. I modeled this after a 3M 603 adapter that I already own, and can confirm that it fits. The 501 style retainer may need to be scaled to ensure a tight fit, or taped in place if necessary. The bayonet connector is a separate part to allow it to be property indexed to the mask. This also allows for additional clearance which is required for a good seal with many respirator styles. Each side will need to be in the correct position to allow it to conform to the mask. I added a single piece design with L and R versions, but these may not allow for enough clearance with many 3M respirators.
***DISCLAIMER*** There has been no laboratory testing done to confirm the effectiveness of this design. It has however passed a OSHA approved qualitative mask fit test using stannic chloride. There is also concern that 3D printed parts are not able to be effectively sterilized due to their micro-porous structure. Use this at your own risk, and only after supplies of other approved PPE have been exhausted.