This is a cap and a lockable nozzle / spout for standard european cartridges for (caulk or silicone) with a threaded mount of M15x2.5
Just in case you have a need for it and your local hardware store already closed. Though a 3D Printer and some filament is neccessary :)
This should help to slow down the drying-out of partialy used cartridges.
Creative Commons Attribution Noncommercial Share Alike