The holders of coal leases issued under the provision of the mineral leasing laws, including chapter 7 of this title, prior to August 11, 1955, or thereafter if based upon a prospecting permit issued prior to that date, upon the discovery during the term of such lease of valuable source material in any bed or deposit of lignite situated within the leased lands, shall have the exclusive right to locate such source material under the provisions of this chapter but the mining and disposal of such source material shall be subject to the operating provisions of the lease and to the provisions of the Atomic Energy Act of 1954 [42 U.S.C. 2011 et seq.]: Provided, That the provisions of this section shall not apply to coal prospecting, permits or leases on lands embraced within entered, granted or patented lands described in section 541c of this title.