This is a Veloci Stainless 3/8" Female Locking QC Coupler.
Locking Couplers were manufactured with safety and speed in mind. These specialized couplers have a small pin installed below the collar that inhibits the collar from disengaging unless you intended on removing an accessory. Stainless locking couplers are excellent for OSHA regulated facilities and food production where chemicals and hot water are traditionally used to clean. Note in the image, the collar can be spun and offset from the pin so that disconnecting an accessory intentionally is the only way it will disengage.
All stainless couplers are National Pipe Thread (NPT) which means they have a tapered Male or Female thread(depending on the model). NPT is mainly used in the Western Hemisphere and was developed on the concept of interference by the third thread. When installing NPT fittings to another NPT thread we recommend that you use 4-5 wraps of teflon tape wound clockwise, or 2-3 drops of Loctite thread sealant.