The screw size is M3x8mm. The FG part number is 6717/8. See the image below for the breakdown.
Here's the link to the PDF assembly guide.
http://www.fg-modellsport-gmbh.de/_P...N/A.8485-5.pdf
I have the 8485 diff and have cammed out one of those screws. If I ever tear down my diff again I plan on using something harder than the FG screws to prevent cam-out.
Have you checked out
http://www.hexcrews.com/ ? These guys sell screws of all sizes that are about as hard as you can buy.