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Push to Connect Straight Union Fitting 1/8" OD: Push To Connect Tube Fittings: Amazon.com: Industrial & Scientific
Duotight Push-In Fitting - 8 mm (5/16 in.) x 1/4 in. BSP | MoreBeer
John Guest 3/8 in. O.D. x 1/4 in. O.D. NPTF Polypropylene Push-to-Connect Reducing Fitting 803159 - The Home Depot