I have a feeling that it is going to be a 5/32 x 36.
In my book of screw demensions looking at every model of savage break open shotguns if the screw is a straight slot counter sunk screw savage used 5/32 x36 screws. If the screw held the trigger guard from the inside of the action into the guard itself savage used a big flathead screw that the book shows as a 5/32 x 36.
If the original trigger guard broke and was replaced with a guard that had the bigger thread of the alloy trigger guard that is a 1/4 x 20.
But by and large that 5/32 x 36 is the break open shotgun guard screw.