The statute of limitations prescribed in section 255(b) of this title shall also be applicable (in the case of a collective or representative action commenced prior to May 14, 1947 under the Fair Labor Standards Act of 1938, as amended [29 U.S.C. 201 et seq.]) to an individual claimant who has not been specifically named as a party plaintiff to the action prior to the expiration of one hundred and twenty days after May 14, 1947. In the application of such statute of limitations such action shall be considered to have been commenced as to him when, and only when, his written consent to become a party plaintiff to the action is filed in the court in which the action was brought.