From the 1940's in solid brass, you will find a block housing just large enough for a steel ball bearing to pass through and intersecting opening allowing for the passage of the solid brass bolt.  Even though bolt blocks passage of the ball it is seen to drop through in full view and instantly showing the bolt is still in place!  Includes the Brema Simplified Illustrated instructions.  Hallmarked in two places with the Brema Interlocking CBS.  Condition very fine.