The 530 COM shared the same drawbar as the 630s, while the non-COM 530's had the same drawbar as the 430. My guess is it had to do with the Independent PTO that was needed for the 530 COM.
The 600B had a smaller/lighter drawbar similar to the 300/350 roundnose. The support mechanism at the rear of the tractor is different for the two designs. - Joe