Ok the mystery of the sideways 'A' is solved. The 5 digite number (Unit number) was actually barred out with a series of sideways A's, virtually all on top of each other with the horizontal barring line formed by the right hand (as you look at an A) but higher of the two sides of the horizontal A. There is one which was obviously a slip of the punch just below the line of As over the serial number which is focused on, but didn't recognise the method of barring out as being related to it. Just had a quick trip to storage today and couldn't resist checking it out.