Not sure exactly what the question is but the piece that is threaded internally on both ends is a coupling. If it were male it would be called a nipple. You seem to be doing this in a rather complicated fashion. Did you consider using PVC
instead of steel? You could eliminate the pieces in question and replace with a glue on threaded female piece. Another place you can probably save some coin is using computer power supply cables to wire the fixtures. The cables come with moulded on plugs and are usually cheaper (comp surplus stores) than the plugs alone.