/usr/lib/tmpfiles.d/xpra.conf is in xpra 2.1.3+dfsg-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | # This is the shared directory where users of the xpra group
# can place sockets if they want to be able to share them.
d /run/xpra 0775 - xpra
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