John Guest Acetal Copolymer Tube Fitting, Straight Adaptor, 1/2" Tube OD x 1/2" NPTF Male (Pack of 10): Push To Connect Tube Fittings: Amazon.com: Industrial & Scientific
Parker Tube Fitting, NPT Tube End Male Adapter - A-LOK Series | Parker NA
Stainless Steel Swagelok Tube Fitting, Male Tube Adapter, 1/4 in. Tube OD x 1/8 in. Male NPT | Tube Adapters | Tube Fittings and Adapters | Fittings | All Products | Swagelok
Picatinny Buffer Tube Adaptor - Midwest Industries, Inc.