Unknown file format, possibly used in conjunction with MSBF and MSBT files. Its magic number is MsgPrjBn.

Use SpeculationEdit

MsgPrjBn is similar to MsgFlwBn and MsgStdBn, With "Prj" possibly standing for project.  In Animal Crossing: New Leaf, only one MSBP file exists. When peeking into the MSBP file with a hex editor, names of MSBF/MSBT files (without extensions) are listed. 

MSBP files likely hold info that organizes MSBF and MSBT files, possibly acting as the game's link to these files.