Answer
The numerals 786 in no way represent ?Bismillaah al-Rahmaan al-Raheem? and is not the Sunnat way. If one fears abuse of the letter, etc. ?Bismillaah al-Rahmaan al-Raheem? should be recited verbally at the beginning of the letter which will suffice for the written script.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai