Holding review of the Board's decision to apply a new rule of law retrospectively is deferential and that the Board's ruling will be disturbed only if it wreaks manifest injustice
Holding issue not exhausted where the "tenor" of petitioner's objection to the Board was "purely factual," but the tenor of the objection on appeal was legal
Reading Board precedent to require that "when a union claims it has attained majority status and the parties, based on that claim, agree to a Section 9 relationship, the employer must challenge that status within a reasonable period of time (six months), or be bound by its agreement"