Do you have any screws that are most likely identical to the one you're missing? If you do, take it to your local hardware store. I like Fleet Farm and Ace Hardware (No idea if you've got those around). A good hardware store will usually have some sort of sizing tool you can try to screw a duplicate screw into until you determine the correct threading. With that new-found information, you can start digging through their inventory for a screw of the same threading and as close as you can get to the same length. Sometimes you've got to worry about the width of the recess it may be going into as well. I know with my IBM F107, I ended up having to use some hex cap machine screws I had picked up vs the stainless phillips pan head screws I would have preferred because the heads on the phillips screws were just a bit too wide.
