A. Not since he left has he written me a letter.
B. Since he left, he has never written me a letter.

Does sentence A have the same meaning as B?
Or it should have been 'not' instead of 'never' in sentence B to make the meaning the same?
They do mean the same but B is natural, A isn't.
A is natural, but it is not very informal. It is carefully composed.

Not since Preston Sturges has a director and producer drawn so much out of such likable but indifferent actors.

Not since the Reds' Pete Rose hit in 44 straight games in 1978 has a hitter come within 15 games of Joe DiMaggio's single-season record of 56 straight. Not since the Rockies' Larry Walker and the Padres' Tony Gwynn, in 1997, has anyone hit.400 even as modestly deep into a season as June 22--until Mauer.

Not since Franklin D. Roosevelt has a new president been poised to pack so many ambitious, costly and, under more normal circumstances, highly contentious projects into one fast-moving bill.
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