Adaptor.
First side:
Thread: 1/4" NPT
Thread Identification: O.D. (approx) 13.5mm (.526") x 18T.P.I.
Second side:
Thread: 3/8" NPT
Thread Identification: O.D. (approx) 16.8mm (.66") x 18T.P.I.
Thread: 1/2" NPT
Thread Identification: O.D. (approx) 21mm (.826") x 14T.P.I.