Figured bass seventh chords

1. What is the figured bass for a seventh chord?
2. What inversion is 643?

What is the figured bass for a seventh chord?

The bass in this case is the fifth of a triad. With seventh chords additional figures are possible: A seventh chord in root position would be 7/5/3 if completely figured. In practice this is abbreviated to just "7." A seventh chord in first inversion would be 6/5/3, and is abbreviated to 6/5.

What inversion is 643?

So your 4-3 chord is just a second inversion chord. These inverted chords tell you only one thing though: what your lowest pitch is going to be. Your root position then is 5-3, first is 6-3, and 2nd is 6-4. The abbreviated versions are: (nothing), 6, and (still) 6-4.