Hi Keith,
They just drive out,you are quite correct they have round heads with a little projection under the head that engages with a cut out in the milled seat that the head sits in .Perhaps warm up the case to 150 to 180c in a domestic oven and then giving the case a firm support give the threaded end a good wack with a hide face or something that wont damage the threads and the round headed bolt should come out. Best of luck
remembering they may have never been removed since manufacture.
regards
Clive