I emailed SMC support, and received the following prompt reply (edited for brevity):
andOur ball cups arms for the Rostock have always been 291.06
I checked the firmware and measured one of our arms. Length on all the ball cup arms for the Rostock Max is 291.06.
I'll wade through the documentation and make sure it gets corrected.
So, they've only been one length for the entire time they've been sold. Hope this helps someone.