quote by frank:
We call that a BX to EMT connector.For running in drop ceilings.We install it just above the t-bars to transition to emt for surface mount switches,receptacles.usually it's AC90 to emt though.Saves time and space
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I don't think that you could use this fitting for BX (AC) or for MC cable since it does not have the retainer for the antishort. I believe that it only listed for 3/8" flex to EMT.