Not sure if this helps as its for 8mm OD line. I ordered 1 for the pressure gauge.
https://www.aliexpress.com/item/Pneumatic-5-16-Tube-x-1-4-NPT-Thread-Female-Tee-Branch-Connector-Push-In-Fitting/32730542052.html?spm=a2g0s.9042311.0.0.31354c4dqH71FD
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