How can I authenticate a letter from President Coolidge to Mr. David Lawrence written on White House stationary dated October 2, 1926.  I think it is a copy but cannot find if and where the original has been preserved.  I talked to the Buffalo and Erie Historical Library and they have a facsimile.  They were not able to find any info on where the "original" letter might be.  If it is a copy, does it have any value?  If it is the original, how do I assign value and how do I sell it?  The letter came to my husband as the executor of his parent's estate and he must prove its value or lack there of.  Thanks for any help you can provide.

Best,

Robin