Many, many ( and several more) years ago when I was doing part of my early training in a Drawing Office I was taught never to use 'off' after a number against a certain item on a manufacturing drawing as this could be (and had been) confused with 'O' being confused with '0'.
For example; 60FF REQD could be confused with 6 OFF REQD and if looked at quickly could end up with 60 being made instead of 6. For this reason I was taught to use 'No' after the figure and to read 6 No REQD.
Of course I'm talking pre the dawn of the computer age when drawings where hand drawn on a drawing board and all notations the same......
Just thought you might like this bit of useless information....................(I'm not expecting any 'likes' btw)
ps All of the above happened at a time when, as draughtsmen, one had to write in a clear and precise manner and spelling needed to be correct (a dictionary was kept in the office, (I kid you not) otherwise the Chief Draughtsman would simply ring the offence with red pencil on a copy of the drawing sent for checking, and throw it back at you!! None of your 'spell checkers' in those days..........................