Thank you gentlemen! Well, I wasn’t smart enough to search the word ‘beau’ separately, which turned out to has its own distinctive meaning, almost the same for both British and American variants… But I strongly suspected that ‘Beaufighter’ might be a kind of ‘beau-tiful fighter’, ‘smart f.’, ‘good looking f.’ or so…

The Beaufort as an etymological origin of B. is also highly appreciated!

Alex.