You need a chill pill Warsaw, I agree with almost every statement here.

What a mess abbreviations have become. This may be my practical side but I personally do believe the "No" is extensively overused, I believe this because it is a very general abbreviation which usually refers to something more specific. Usually a number should represent something like an index, line, position, etc. and in most situations it should be appropriate to use a more descriptive title. In technical applications like Databases titles, Spreadsheet or Table headings, etc. I would urge against this temptation. I'm guessing it became popular back in the 60s and 70s every big name ended in Co.

I would only use the No abbreviation as part of a name or quote where the context is clearly stated, as in "Bus Stop No 3".

I see this is an old thread, however I feel the need to introject being an engineer.

I expect the desire to pluralize stems from the fact that we are often working with multiple fractions at a time (eg 3/4), however we would be less inclined to want to pluralize 1/4. However the numerator is not the subject of this conversation, it is the unit of that number we are discussing. Strictly speaking I would also never encourage a student to pluralize any measurement as the unit is just that; one unit.

To elaborate 1/3 of two inches would actually be 2/3, which is clearly not the intention being communicated. I do agree with Ursus however, in any practical environment (and any guidelines) the deprecated sizes of late British royalty's body parts should no longer be used as units of measure in a competitive global environment.