Those screws are typically staked to keep them in place. Basically a chisel mark to expand the end of the screw that sticks thru the throttle shaft, making it more difficult to back out. But nowadays, some permanent threadlocker does the same job. Yours look like they sheared off for some reason. Maybe the last rebuilder of that carb overtightened them or they just got weak from age. Like Dan said, they may have gotten sucked into the engine, but probably got spit out thru the exhaust. Being made of brass is a plus as it's a soft metal and won't cause a lot of damage bouncing around in a cylinder.
