Screws.
We don't know how O.S. decides upon what type of bolt/screw to use. Possibly availability and price. Also the crosspoint carb retaining screws don't protrude as much as a socket bolt.
The main problem in dealing with the carb screws is the fact that American Phillips screwdrivers are too pointy. We've have to blunt the nose of a #2 Phillips screwdriver on a grinding wheel so that it fits the head of the screw properly. With a properly-fitting screwdriver, they come out easily.