http://irefindex.uio.no/wiki/README_MITAB2.6_for_iRefIndex_9.0#Mapping_to_Legacy_RIGIDs
The main legacy mapping should now cover all non-canonical RIGIDs in
iRefIndex 9.0. Previously, it only covered canonical RIGIDs. I
apologise for any inconvenience caused through the use of the
incomplete version of the file.
Paul