The two insulator threads are 8" apart, the centre bolt is 4⅛" long suitable to pass through a pole arm.
The Spindle No.10 is normally used for carrying insulators on both sides of a pole arm, on an intermediate pole. On a terminating pole a type No.8 would be used instead.
Its use in a wall bracket is more unusual. Probably not intended for this configuration as the securing bolt sticks out well above the wall bracket. Wires from poles to buildings are usually separated vertically, not horizontally.