According to this diagram:
Numrich Gun Parts Corp. - The World's Largest Supplier of Firearms Parts and Accessories
the rib is held in place by two screws -- one in front of the front sight, and the other one at the rear of the rib. Also according to the diagram, they are the same screw (both labeled #54). If your gun still has one of these screws in place, I'd recommend carefully removing it (or having a gunsmith do it for you), and then having it measured. Most well-equipped machinists have a set of screw thread gauges, which have teeth that can be compared to the threads on a sample screw until they find one that matches. With the thread type, pitch and TPI (number of threads per inch) identified, and the overall length, diameter and head size/shape, you could go "screw shopping" with confidence that what you buy will do the job. If you have a good local source for fine-thread screws, you might be able to take it in and ask them to match it for you. Most gunsmiths have a "spare screw box" with a bazillion screws in it, and if what you need is a common size and type, he might have one on-hand. I wouldn't bet on this happening, but it's possible; the question is, can he find/match it if he has one?
Another thing to consider: the hole where the missing screw goes might have the threads "stripped", meaning that the hole is ruined for that size screw. Even if you find a replacement, it might not stay in place well enough to do the job. A good gunsmith can carefully re-drill and tap (thread) the hole for a slightly larger screw which will get the job done. In the long run, it might be less trouble to just have this done rather than search for an original screw, as most gunsmiths have the stuff to do this type of job on-hand in their shop. Be aware that a good gunsmith's time is not cheap; expect to spend $60-$100 per hour of work he does on your gun.