It'll be this year's car again, with very limited changes. This was mandated by the FIA as part of the COVID measures that also saw the introduction of the new technical regulations pushed back a year (for which AM won't be able to copy Mercedes)
It'll be nice to have some 'b' cars used again, would the last one have been the old FW14b?
Pretty much I think. Unless someone like Spyker or Marussia did one when they were running out of money.
Ferrari had the 412 T1 and T1B in 1994, but that was a B spec in a different sense, i.e. a necessary redesign in season, rather than just being developed and raced again for another year. The original 412 T1 had those beautiful sidepods, but beauty did not necessarily translate to performance!
There are many more examples of B cars in-season but rarely when the car was so good they used it again next year, in modern times anyway.
McLaren effectively ran with a B car once or twice around the turn of the century when they were dominant (and the planned successor car was comparatively rubbish), at least to start the season, but never called it "B", just either the same or the next increment of the MP4 series.