Sargent & Greenleaf sold a line of zero-change safe locks (6770/6780 series) from about 1964 to 1992. In their catalogs it was stated that these cannot be used with dial rings which have a change mark. I’ve always wondered if there was an actual reason for this, or if they overstated the situation (rather than saying must not or should not) just to discourage any confusion that might arise with a zero-change lock and a ring that has a change mark. Does anybody know?
