It was an early pattern design for the new coinage. It was obviously decided to go for a beaded approach both sides, perhaps after seeing these few pattern strikes? They probably struck a handful and melted (most of) them down. You could understand continuing with the old reverse, but as there was no demand for pennies between 1949 and 1961 (1950-51 was West Indies, 1952 and 1954 were VIP/experimental issues only, and the entire 1953 production run was for the sets), they probably thought they had plenty of time to play around. After all, George VI died early 1952 and the Coronation wasn't until mid-1953, so there WAS plenty of time.