The Insulator Spindle No.13 is normally used for carrying a pair of wires mounted on the top of a pole with a saddle bracket. This was the accepted practice if there was insufficient space on the pole arms. There is a plain central hole for a bolt to secure it to the bracket.
This very usual configuration of two brackets No.13, carrying 4 insulators on the top of a 'D' pole has been replaced since the photograph was taken in March 2014. It use in a wall bracket is questionable too.
The two insulators threads are 9" apart, there is a single hole in the centre of the spindle for it to be bolted to a bracket.