From an earlier PICLIST thread on this list:

Smaller die, better yield, lower cost.
Less capable pin drive (lower current than A). Some circuits that
use the full drive spec have had problems. Check the archives for
the discussion. Early May IMS.

Nicholas Irias wrote:
>
> Does anyone have a reference indicating what the differences are:
>
> PIC1673A vs. PIC 16C73B
>
> and
>
> PIC1674A vs. PIC 16C74B
>
> The "B" parts seem cheaper (at least the EPROM or JW versions) at digikey.
> Is there some drawback to the "B" part?
>
> -Nicholas