I don't know the size and pitch of the thumb screw, but I do have this link from a Lumberjock's thread that has information on size/pitch for Stanley/Record bench planes. It might give you a clue as to what is the thread for your missing thumbscrew.
There are a couple of links in that thread with lots of info on thread/pitch sizes used and the different standards.
If you can't find a fit with a standard bolt, you might try using some of the screws/bolts from bench planes you have to determine if one of them fits. If one of them does, you could then at least find out the size of it and maybe cobble something together from spare parts.
St. James Bay Tool Co. does have a selection of Stanley reproduction replacement parts available, so you also might try contacting them and see if they could help you.
A forum community dedicated to professional woodworkers and enthusiasts. Come join the discussion about shop safety, wood, carpentry, lumber, finishing, tools, machinery, woodworking related topics, styles, scales, reviews, accessories, classifieds, and more!