Any mining claim located in a manner prescribed by the mining laws of the United States upon lands of the character described in section 541 of this title, prior to May 25, 1955, if based upon a discovery of valuable source material contained in lignite shall be effective to the same extent as if such lands at the time of location, and at all times thereafter, had not been classified as or known to be valuable for coal subject to disposition under the mineral leasing laws, subject, however, to the provisions of section 541 of this title: Provided, That no extralateral rights shall attach to any mining location validated under this section: And provided further, That the locator or locators of such a mining claim shall, not later than one hundred and eighty days from and after August 11, 1955, post on the claim and file for record in the office where the notice or certificate of location is of record, an amended notice of the mining location stating that such amended notice is filed pursuant to the provisions of this chapter and for the purpose of obtaining the benefits thereof; and that a copy of said amended notice is, within the said one-hundred-and-eighty-day period, filed in the land office of the Bureau of Land Management for the State in which the mining location is situated, and the mining locator thereafter complies with the requirements of this chapter.