The following information is taken from pages 179-182 of this work by Michael Salter and concerns the four documents constituting Nuremberg exhibit PS-501, discussed in this series by Hans. The first involvement of the OSS was to locate the documents among those held by British intelligence in London. A cable from Donovan to Jackson dated June 1, 1945, detailed the role of Colonel Amen in obtaining "documents (originals) which contain details of gas by "death van" fitted out by that purpose" [Salter p.180, citing cable 18124, 1/6/45, Jackson Papers, Box 101, Reel 7, my emphasis]. These were placed in a dossier prepared by OSS X-2 branch and eventually picked up by Whitney Harris, who discusses here (pp.198-199) how he used them in the interrogation of Ohlendorf while preparing for the case against Kaltenbrunner. The documents were also authenticated by Rauff in Ancona, as Hans shows here. Although the Becker-Rauff document became part of PS-501, it was originally filed by the OSS as Feldpostnummer S2704 / SECRET. Rauff gave a further very detailed confirmation of the document in Chile in 1972, shown here.
On the whining of Weckert and others about the origins of these documents, I refer readers to the quote by Zimmerman in Roberto's post here.