Old Style Rega Arms

I would email Rega and ask them.

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.

I got the RB300 and it dropped right in. And in fact the mounting nut from the RB200 fit the threads on the RB300, it was a perfect fit.