it’s quite hard to help you on that one if don’t provide a filename or where you encountered that string.
As the community is small, this likely refers to Ana Gibbons’ PhD and related papers on reconstructing the Indian Ocean and surrounds: https://ses.library.usyd.edu.au/handle/2123/8580. Although “MS12” could refer to one incarnation of M. Seton’s 2012 changes to the rotation model.
This is a case where a more verbose format of the rotation file such as the GROT rotation file format would help (as in providing a link to a certain model/paper/thesis or a longer description).