I would email Rega and ask them.
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I have read that they are compatible. I'm planning on replacing the S-shaped arm on my old Planar 2 with an RB300 in the next few days when it arrives in the mail. One thing I found was the mounting hole for the S-shaped arm is 25mm and the newer straight arms use a 23mm hole diameter. There is a stepped adapter available to center the new arm in the larger hole.