/lib/udev/rules.d/40-libxenomai1.rules is in libxenomai1 2.5.5.2-1ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | # real-time heap device (Xenomai:rtheap)
KERNEL=="rtheap", MODE="0660", GROUP=="xenomai"
# real-time pipe devices (Xenomai:rtpipe)
KERNEL=="rtp[0-9]*", MODE="0660", GROUP="xenomai"
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