Many thanks to Martyn Pollard who took the trouble to post me a used Fein blade.
Both blades have the same size centre hole, so each will 'fit' the other in that respect.
The differences are that the Fein blade has 8 cut-outs in a star shape, whereas the Bosch has 12 holes. The Bosch blade holes are on a larger PCD than the Fein cut-outs.
John Rumms idea that using an old blade cut down as a spacer, in order to make the studs flush would therefore work, but fitting a Bosch blade to a Fein, or a Fein blade to a Bosch would rely on getting the centre bolt tight enough to avoid play.
The Bosch studs fit flush with the blade surface, don't know how the Fein works - are they flush as well?
So in short, they are not interchangeable as standard, but with an old blade as a spacer and enough welly on the bolt, a bodge is possible.