None of the above caused my grief.
I have a NOS rear handguard with some recently applied oil finish. The ring had rotated inside the stock when I started turning the takedown screw. I backed it off, and got the ring set up properly. The break happened when I rotated the handguard into the ring. The ring broke at the two furtherest angles, where the band bends at sharper than 90-degs. Both sides let go. Then I had to dismount everything and fish out the bottom half. To reinstall, I unscrewed the ears and the foresight blade, then slid the new ring into place. I set the handguard in position without the foreend, and reassembled normally.