Where, either upon review of a decision rejecting a claim for pension or following a decision of the President of the Pensions Appeal Tribunals that the decision of a Pensions Appeal Tribunal be treated as set aside by reason of the availability of additional evidence, an award is made on the basis of evidence contained in documents produced by the claimant which were not available to the claimant at the time of the decision which is subject to the review or appeal because those documents were classified at the time, payment may be made in respect of a period commencing on, but not in respect of any period before, the date three years before the date of the application for review or, as the case may be, application to the President of the Pensions Appeal Tribunals that the decision of a Pensions Appeal Tribunal be treated as set aside.