A letterbox lock is a type of tongue lock that is a lock named after the application.
There are four common size specifications:
1. D18.5 x L16 means the lock cylinder has a diameter of 18.5mm and the working length is 16mm.
2. D18.5 x L20 means the lock cylinder has a diameter of 18.5mm and the working length is 20mm.
3. D18.5 x L25 means the lock cylinder has a diameter of 18.5mm and the working length is 25mm.
4. D18.5 x L30 means the lock cylinder has a diameter of 18.5mm and the working length is 30mm.