What is the size thread for the little screws that hold the pins in on a Francotte falling block action?

If in doubt visualize a BSA Martini Cadet rifle action and take off the kangaroo.

You will need to take it to a doublegun gunsmith and have him look at it for you. Be wary of taking it to just any "gunsmith." Your typical local gunbutcher can screw up a double while trying to do the most basic of tasks. True doublegun gunsmiths can tell you horror stories for hours.