The directory in .pants.d that pants chooses to use for a valid target corresponds to the sha1 of the cache key. In order to point at an "old" but valid cache key, I'm guessing it is supposed to point the

current

symlink 'back' to the old directory but never does. Sounds like you think I'm insane, so I'm going to fish through

cache_manager.py

and see what I can find.