The "small" b was used as a 6, or turned upside down, a 9, by Buick. Probability is your spare is a 1951 by the number. Pre ww2, the first digit, 6 in your case, denoted series. If that is the case, 6 3,329,964 is a 1937 320 engine. Later, the last digit is the series marker. So 6,332,996 4 is a 1951 263 from a Special. does the engine in question have a pad about 1/3 way back from the front with 4 bolt holes, each side? If so, for sure 1948 or later.