I think it stands for "Notable". It may also be translated as a B in certain contexts.

I'm only giving my answer a medium confidence level because you might not like my suggestion, although I think it leaves no room for doubt.

I translated a secondary school transcript a couple of days ago I decided to use a different formula. I translated words like 'pass', 'good', etc. and left 'notable' as it was. At the bottom of the page I wrote:

Pass = 5.0-5.99; Good = 6.0-6.9; Not. = Notable = 7.0-8.9; Excellent = 9.0-9.9; Honours = 10.0

I did this because some years ago I needed a sworn translation of my university transcript and 'Merit' was translated as 'Aprobado con mención', which I don't like at all. I think 70% is more than just a 'pass with a mention'.

The word 'notable' exists in English, although I admit I've never seen it used in this context, but I decided to use it anyway.