Looks like the stock wood on your gun is walnut , or some other dark stained wood. Fix #1 would be brown stained epoxie to glue a piece of walnut to fill the cavity , then a little dark stain and finish. Fix #2 would splice a piece of brass the same thickness as the tail of the original trigger guard. A filler splice uses a thin backer plate soldered under the tail of the original guard , then clamp and solder the new brass onto the top of the backer plate. Careful joining of the splice , will be almost invisible. Trim , shape and inlet the new tail , and done. I've used both of these methods on restorations of originals many times w/success. Epoxie stain can be accessed from Brownells , and some woodworking stores. Doesn't take much stain to match the color of any wood , just a touch on the tip of a screwdriver blade.