Just a Guess.
If it works in a similar way to a gas 4 stroke engine (i.e. Honda for example) the spring will be on the cover side with the narrow side towards the cover and wider side resting on the diaphragm.
The fuel pressure will act against the spring by pushing the diaphragm towards the opporite side (cover) and the spring will act to restrict the flow of fuel (hence the regulator affect).
I have seen many regulator diaphragms on Walbro's, Zama's and the likes but never on an OS 200P, a picture is worth a 1000 words...