1/4" UNC grade 5
Since you haven't got a reply yet, I'll take a crack at it. I have a different fuel servo on the IO-360M1B I purchased from Van's, however I suspect things are similar. They are a 1/4" UNC thread, and if you look at your engine you will see that many of the 1/4" UNC bolts (like the ones that hold fuel spider bracket to the crank case and the baffles to the engine) have SAE Grade 5 markings on them (the three lines coming out radially from the center of the bolt head). You should be able to get these locally in Europe (I can find them here in Tasmania, so I'd say your chances are pretty good). Don't forget you will need some type of locking device on the bolt (star lock washer/drilled head with lock wire etc) which you will have to look into.