Lots of these to go around, here's my take. Fits PTFE tubing commonly used for 1.75 mm filament (4 mm outer, 2 mm inner diameter). The front part of the PTFE fitting base is threaded with DIN pipe threads, and fits these connectors:https://www.aliexpress.com/item/10PCS-Pneumatic-Connector-PC4-01-1-…