I have 2 M8 bosses that are not quite a matched pair - A bit difficult to describe, but one is threaded right up to the shoulder where the brake sits, the other is slightly longer with a small unthreaded portion near the shoulder. They were working fine on my Manitou Mars but when I swapped the brakes to M950s the spring didn't quite reach the hole it slots into on the longer of the bosses. I can send photos later today, £3 posted if you want them.