I've been and had a look at mine to see if I can help. The screws you need are countersunk (to fit flush with the spurs and the skates) and also very short - too short to measure the threads. However, my tin of miscellaneous small screws yielded one which fits the hole. It's 36 threads per inch and 0.154" diameter, which I guess is probably a nominal 5/32" (=0.1562"). You're welcome to it if you want, but it's cheese head, so would not be much use, and I only have one. It's not a standard Whitworth size as far as I can see, but the fact that some time in the last 50 years I salvaged a screw of the same size must mean it was not a thread exclusive to plane makers!
At this point, the usual advice is to point out that the spurs on combi planes are often not much use when present, and suggest knifing your lines before planing, should you ever need to make cross-grain cuts.
