No idea of the purpose of that o-ring… probably to help center the forend or to dampen the plastic on plastic? Either way, I haven’t had problems that ring but I have had problems with the plastic ring in front of it, part 68.
The first time I disassembled a and reassembled the gun, the forend jammed up so I pushed a little too hard and the part popped off. Now whenever I go to seat the forend, once the mag tube makes contact with that ring part, I place my finger in the tube and gently ease it away from the barrel. The forend then drops in place.