Roger doesn't use a Breguet overcoil on his hairsprings for best isochronism. I have spoken with Brandon1 about it and we can't figure out why, given concern for thickness clearly isn't the issue. It's something missing on a few high-end watches and watch companies. Sometimes there is a reason, sometimes it's a head scratcher; sometimes watchmakers are open about it (the Gronefeld brothers were super nice in explaining), sometimes not at all--Roger's ignored polite inquiries by myself and others.