/usr/share/igtf-policy/classic/SDG.namespaces is in igtf-policy-classic 1.59-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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#NAMESPACES-VERSION: 1.0
#
# @(#)c48c63f3.signing_policy.namespaces
# CA alias    : SDG
#    subord_of: 
#    subjectDN: /DC=CN/DC=Grid/DC=SDG/CN=Scientific Data Grid CA
#    hash     : c48c63f3.signing_policy
#    (generated automatically from c48c63f3.signing_policy)
#
TO Issuer "/DC=CN/DC=Grid/DC=SDG/CN=Scientific Data Grid CA" \
  PERMIT Subject "/DC=CN/DC=Grid/DC=SDG/.*"
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