I did some more investigations on the kernel version on the 3.8.1 CD.
The kernel identifies itself as (uname -a output):
Linux Knoppix 2.6.11 #8 SMP Sun Mar 20 21:09:51 CET 2005 i686 GNU/Linux
The kernel is part of a custom version of the Debian kernel-image-2.6.11 package.
The first Debian version of kernel-source-2.6.11 is later: 2005-03-21. So probably Klaus (?) used
the kernel source directly with Knoppix specific patches included.
The Knoppix 3.8/Cebit 2005 version was based on 2.6.11-rc3 according to the article (in German)
on www.heise.de/ct.
My guess is that we still have the same version in 3.8.1.

My problem with usb_bulk_write in libusb was introduced with kernel Changeset 1.1938.455.15 on 2004-11-27
and solved with Changeset 1.1982.126.8 on 2005-02-25. See the kernel Bitkeeper logs on
linux.bkbits.net. This corresponds to 2.6.11-rc1 to 2.6.11-rc5 inclusive.

So my question remains:
Can anybody confirm that the 2.6.11 kernel of Knoppix 3.8.1 is derived from one of the 2.6.11-rcx versions?