Any existing valid contract made and approved prior to June 26, 1936, pursuant to any Act of Congress by any tribe, band, or group of Indians with an attorney or attorneys for the rendition of services in the prosecution of claims against the United States under authority of which suit or suits have been filed, and which contains a limitation of time for the completion of the services to be performed may be continued in full force unless a subsequent contract dealing with the same subject matter has been made and approved.