As pictured.  This is interesting.  It is an old solid brass door knocker.  It appears to have been improvised.  It looks like someone took an old rail spike and must have drilled a hole through the end and then a nail through that to attach it to the knocker portion.  They used the smaller spikes with the narrow gauge rails.  If your door is 2 1/4 Inches thick, you will be all set.  If not, you may have to further improvise.  This is old, it was so black that I thought it was cast iron, however it didn't respond to a magnet so I cleaned it up a bit to reveal the brass.  You could polish it to a shine or leave it with it's patina.  It is a great old piece.