Out of 3 Mig 31 models available in CA (not sure why Mig-31A got removed...) Mig 31BM has the oldest model dated back to 2008, which has problem casting shadows and is over-polygoned.
This package contains models changed late 2010 with multiple level of details and correct shadow casting. So that you can finally ditch the SHD file.
Original creator : jv44
Please note the new model is not compatible with any previous skin. This update is released as-is.
You will need to download & install Mig31m/bm pack from Brain32, then update with this mini pack. Find Brain32's file here