No, B9 is not the accepted answer. If you would take the time to do some research you will find that B9 is not the starting number.
You will also find that B10 is in the first production R3 Avanti, R-5089.
The roster product is in three parts.
First, there is a listing of engines by "B"-number. Every B-engine number that we believe Paxton used, is on this listing. Those numbers run from 1 to 117 plus 153, 299 and 300. The most current information we have on every B engine is shown in one row. There are many blank rows, indicating that the engine has never been found or has not been reported by B number. The data for some engines are very fresh and some is 30 years old.