Hoess at Nuremberg:
"In Birkenau there were five installations - two large crematoria, each of which had a capacity for receiving 2,000 persons in the course of 24 hours. That is to say, it was possible in one gas chamber to put to death up to 2,500 persons; in five double ovens heated with coke it was possible to burn at the most 2,000 bodies within 24 hours; two smaller installations could eliminate about 1,500 people, with four bigger double ovens to each of them. Furthermore, there was also one open-air installation - that is, an old farmhouse was sealed and turned into a gas chamber, which could also contain 1,500 persons at one and the same time. The incineration was carried out there in an open pit on wood, and this was practically limitless.In my estimation, it was possible to burn there, in 24 hours, up to 8,000 persons in this way. Hence it was possible to exterminate and eliminate up to 10,000 people within 24 hours in the installations described above. As far as I am aware, this number was attained only once in 1944, when delays occurred in the arrival of trains, and consequently five transports arrived together on one day."
Hoess was the guy who "told" FG the furnace capacity. He also "told" Eichmann this was the figure
"In Auschwitz I had to look at those installations yet once more. I told Hoess this and as a consequence he ordered a field car. We rode in the field car through some area - I didn't know my way around in Auschwitz. This was a section - far from the headquarters. I had been there on my business several times only in the headquarters, near the main entrance, I had never been further inside - I also did not have any inclination to do so - and then I saw large buildings, large buildings, this was already in the guise of a factory, the enormous chimney, and Hoess said to me: "Yes," he said, "here there is a capacity of 10,000 - yes 10,000."..
Clever conspirators. It is extremely likely they introduced this element into the story to strengthen the FG document, of which they knew the typewriter matchable carbon copy would surface sooner or later (in this case decades later).
It also probably doesn't need to be said that it hardly matters if these documents are actually "accurate" to any particular story. The Becker Gas Van letter, for example, has nothing about it that screams forgery or is "inaccurate" yet it is clearly a forgery, because as we know the Gas Vans story is false. The conspirators tracked down FG's war time typewriter, or found an identical defective typewriter - a trivial task. It would be likewise trivial for them to produce any such "accurate" document.