Breitman's account reads as follows:
The British received a major new source of information about past SS and police activities in the Soviet territories when, in October 1943, a plane carrying a Croatian Air Force captain and a German police escort landed behind British lines in Allied-occupied Italy. It happened that the German policeman, an Austrian named Robert Barth, had served previously in Einsatzgruppe D (Einsatzkommando 10b) and was frightened enough to talk about it. (Only the initial summary of Barth’s comments to British intelligence has been released so far.)I decided to order the 1943 British summary [HW 16/1, pp.47-49] and the 1946 Nuremberg affadavit [NO-3363, ordered as a Yad Vashem copy under reference O.18/293] to make my own comparisons. The order for HW 16/1, pp.47-49 failed because the interrogation could not be located but I had greater success when I re-ordered it, based on Irving's reference, at HW 16/9. A scan of the first page can be found below and reveals that this was a preliminary interrogation that would be followed by a more detailed one using the questionnaire that had just been received. It also noted that Barth was keen to supply as much detailed information as possible. The text of NO-3363 is also extracted below and shows that Barth gave a detailed account of Einsatzgruppe D activities in December 1943. Based on these two sources, I would conclude it to be highly likely that Barth gave his gas van details to the British in that December 1943 interrogation.
Barth described the formation of the Einsatzgruppen in May 1941 and their tasks—fighting partisans and Communism and carrying out general intelligence duties. He admitted that commissars and leading Communists were arrested and shot but misleadingly claimed that the Order Police (“Schupos”) and Waffen-SS did the shooting. (They did, but the Einsatzgruppen did even more.) Barth revealed that Jews were almost invariably shot and in later stages gassed. He then discussed the 1943 activities of Einsatzgruppe E [sic] in Serbia and Croatia [see note 60]. After the war, he gave American interrogators interested in prosecuting the heads of the Einsatzgruppen a much more detailed picture of events in Germany and the Ukraine in 1941 [see note 61], but it is unclear just how much detail he offered the British in late 1943.
....60. Summary of interrogation of Barth, approximately mid-Nov. 1943, in PRO HW 16/1, pp. 47-49.61. [NO-3663, October 1946]. See NA RG 238, Microfilm Series M-1019/R 5/240ff
Moreover, the fact that these details were not "contaminated" or "coached" can be confirmed by the sequence of events described in HW 16/9, and in the tone used. The sentence on gassing is not highlighted or foregrounded in the text. Barth does not give the killing of Jews as being one of the three main tasks of the Einsatzgruppen, which in itself is chilling because it would mean that extermination was carried out as a matter of course, without needing to be spelled out as a main goal. It was simply a taken-for-granted assumption by Barth, as recorded by the British, that it was perfectly normal that Jews were "almost invariably shot or in later stages gassed"; so commonplace and embedded that it did not need to be emphasized. The British made no attempt here to feed Barth information that conformed to a British propaganda line of that moment; the interrogation was clearly designed purely to obtain information, not to produce misinformation, and its existence remained secret until the 1990s.
Figure 1: Extract from Report of interrogation of Robert Barth, undated [but some time between 1-15 November 1943], London HW 16/9:
Figure 2: Extract from Interrogation of Robert Barth, probably 8.10.46, Sandbostel, NO-3663