AT329B does not exist, I think you mean AT320B. Tip: Research without the spacing. 😊
I agree with bdoudard: B is probably the newer model. I don't think there are any significant improvements over the AT320A.

The AT 329B version is perhaps a slight technical improvement on the AT320A, in general on the Kenwood site it is indicated, but I have not found the AT 329B.

To possibly answer this question, I had to google it myself and visually see no difference.
I would ask Kenwood if I were you. Possibly different article numbers for the different food processors?
I have my own Kenwood Chef AT 320 A and it works perfectly.