Proline Series 5/16-in x 5/16-in Barbed Splicer Adapter Fitting in the Brass Fittings department at Lowes.com
1/4"-20 to 5/16"-18 Air Cleaner Stud Adapter
1pc Brass R410a Adapter Connectors 1/4" Male To 5/16" Sae Female Charging Hose To Vacuum Pump For Air Conditioners Mayitr - Pneumatic Parts - AliExpress
6AN Male x 5/16" SAE Quick-Disconnect Male Adapter With Zinc Finish - Russell Performance
1 Brass Brake Line Adapter 5/16" x 3/8" x 5/8" x 18 TPI