From "Dark Sun - The Making of the Hydrogen Bomb" by Richard Rhodes, Simon & Schuster, New York 1995, ISBN: 0-684-80400-X. (Also see "Betrayal at Yalta.")
Chapter 5 "Super Lend-Lease" - on pages 99-100 - list of nuclear materials (tens of tons) sent to Stalin in 1943. Note that the Trinity Test was in ... 1945.
Oh, yes, Harry (the Hop) Hopkins (as in Agent 19 via the Venona Project) was the head of the Lead-Lease program at the time.
FDR sure could pick'em!
Recall Ted Hall, the Rosenbergs, Fuchs, ... "Quantum" ...