privileges and to their full backwages, inclusive of allowances, and to their other benefits or the monetary equivalent thereof computed from the time their compensation was withheld from them up to the time of their actual reinstatement. Should reinstatement not be possible, private respondents are ordered to pay petitioners separation pay as provided by law. Anent petitioners contention that they are entitled to retirement benefits under the optional retirement policy, we are constrained to uphold the findings of public respondent NLRC. A perusal of the records will reveal that the NLRC did not err in denying petitioners claim under the optional retirement policy allegedly announced by Captain Inocencio Estaniel at the General Assembly held at the Army and Navy Club sometime in 1977. The evidence of petitioners regarding the supposed announcement by Captain Estaniel of the controverted optional retirement plan which consisted merely of the affidavits of petitioners and their witnesses was successfully rebutted by the evidence adduced by private respondents. Furthermore, nowhere in the CEIP24[24] is there a reference to the alleged optional retirement plan, nor is there a provision for retirement upon service of 20 years in the company. Having failed to substantiate their allegation that indeed Captain Estaniel announced this company policy on early retirement in 1977, petitioners cannot, thus, successfully invoke the doctrine of estoppel against private respondents. Regarding petitioners allegation that public respondent NLRC seriously erred in invoking the parol evidence rule against petitioners as there is no written agreement on optional retirement so as to make this rule applicable, we find the same to be without merit. Contrary to the allegations of petitioners, provisions on retirement benefits are specifically embodied in the CEIP which was part and parcel of the contract of enlistment signed by the petitioners. Moreover, we note that petitioners are in fact anchoring their claim for retirement benefits, in the alternative, under Section III, paragraph (c) of this same CEIP. Hence, they cannot validly deny the existence of the provisions on retirement benefits, and rely merely on the alleged unilateral issuance of private respondents. The above notwithstanding, petitioners can nevertheless properly claim 100% of the total amount credited to their account under Section III of the CEIP,25[25] as well as paragraph 2 (h) of the Memorandum dated March 9, 1977.26[26] The Consecutive Enlistment Incentive Plan or CEIP provides, among others: (a) that when the employment of an employee terminates because of his retirement (with sixty (60) years being the mandatory retirement age) , death or permanent and total disability, 100% of 24[24] Rollo, pp. 170-173. 25[25] supra. 26[26] supra.