Effect of finality of finding of claims adjudicator, referee, or board of review – estoppel. If, in any “finding” made by a claims adjudicator or in any decision rendered by a Referee or the Board of Review, it is found that the claimant has been paid wages for insured work by any employing unit or units in his base period, and such “finding” of the claims adjudicator or decision of the Referee or the Board of Review becomes final, each such employing unit as shall have been a party to the claims adjudicator’s “finding” as provided in Section 701, or to the proceedings before the Referee, or the Board of Review, and shall have been given notice of such “finding” of the claims adjudicator, or proceedings before the Referee or the Board of Review, as the case may be, and an opportunity to be heard, shall be forever estopped to deny in any proceeding whatsoever that during such base period it was an employer as defined by this Act, that the wages paid by such employing unit to the claimant were wages for insured work, and that the wages paid by it for services rendered for it by any individual under circumstances substantially the same as those under which the claimant’s services were performed were wages for insured work.