I don't fully understand what series is supposed to be used for. There doesn't seem to be one feature that the posts share.
Should it be aliased to invalid tag or just invalidated? Aliasing to disambiguation is probably off the table, since series is most likely incomplete instead of meaning multiple things.
Actually looking closer, it might be related to sequence, but there's enough posts that aren't really sequences that it'll be messed up if an alias is used.