Thank you, SG
I got the chrome scalloped ring off and the lens apart and I eventually found that none of that was necessary because the ring I needed to remove was not affected by anything internal.
It was held in place by little ridges at the internal ends of the "teeth" going inside the shutter. It was just a matter of levering one of the teeth away from the sides of the outer ring with the engraved shutter speeds, rather than trying to force the ring off directly, and the offending ring popped out.
See the attached photos for closeups of the ring in place and of the ring removed to show its internal "teeth".
The photos have been "enhanced" to show details. Both rings are normally solid black.
Hope this helps somebody!
Roger