I bought this for a pittance on eBay recently. It was advertised as an authentic signature of older FDR's half brother James Rossevelt "Rosy" Roosevelt (1854-1927), which indeed it is. Rosy was also the father of FDR's slightly older and somewhat strange and embarrasing half-nephew and short-lived Harvard Classmate Taddy, who may have been the nidus for the bitter disappointment of FDR not being named to the premier Harvard society, The Porcellanian.
The document in question is a fragment of a 1925 legal document, perhaps his will. The most interesting aspect of the document. wholly missed by the seller, was the witness, E.R. Roosevelt, none other than FDR's wife Eleanor Roosevelt Roosevelt. This is surely a more valuable signature than the one featured in the sale. It also completes the rather curious grouping of signatures with two Roosevelts in it!
I have been kindly invited to attend a lecture on October 18th at the Lahey Clinic concerning the Lahey Memorandum.The post-lecture Q and A should be most interesting! As I have said, FDR probably had a secret middle-of-the-night operation there for his metastatic cancer (open and close by all reports).