There are two types of side plate screws on a modern S&W revolver. On these guns, the two that are visible with the grips in place are the same screw. The yoke has been fitted to the one just in front of the trigger. The fitting involves altering the yoke and not the screw so in theory the two screws are interchangeable. In practice, S&W uses screw machines that were made in the 1940's so there is some variation from screw to screw. This means that if you replace the yoke screw you might end up with an end shake yoke problem. On newer smiths, there are three different screws. The yoke screw has a spring and plunger which engages the yoke. These were designed to save the step of hand fitting the yoke.
On these guns, the screws are obviously not interchangeable. You can tell by removing the yoke screw to see if it has the plunger. The third screw is the one under the grips and is a flat head screw instead of a round head screw like the other two and is not interchangeable. So, depending on what model of revolver you are working on, you could have as many as three different screws holding the side plate on. (A four screw gun also has three different side plate screws.) I know this may sound confusing, but it is necessary that you keep in mind that there are several different screws when you order your new ones.