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Sorry to harp on about it, but please check this website http://store.advancedmart.com/neopbpc26biw.html where you will find your exact sentence, but with the word "ground". "Grinned" has one meaning only, and that's "smiled broadly".
69 websites seem to have discovered the mistake in the original document, and you can find the exact sentence with the correct word "ground", eg: "The precisely ground lenses produce..." www.gunaccessories.com/NewconOptik/ Binoculars&Lenses/Binoculars.asp
Hi Hamid - I believe Fuad is right, although you declined his answer. There are 205 search results on Google for "precisely ground lenses". There are only 22 results for "precisely grinned lenses" and ALL 22 sites quote the same sentence in your question!