/usr/share/doc/libsnmp-ruby1.8/yaml/IPV6-FLOW-LABEL-MIB.yaml is in libsnmp-ruby1.8 1.0.2-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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--- ipv6FlowLabelMIB: 1.3.6.1.2.1.103