This is a National Dock Labour Board St John Ambulance badge, a rare vintage enamel pin typically dating back to the 1950s. 

It represents a historic partnership between St John Ambulance and the National Dock Labour Board (NDLB) to provide essential first aid and medical services at Britain's ports.

Key Details & Significance

The Organization: The National Dock Labour Board was established in 1947 to provide security and regular employment for dock workers, who previously faced dangerous and unstable working conditions.

Medical Care: Port work was high-risk; the NDLB collaborated with St John Ambulance to run Port Medical Centres and train volunteers to handle accidents and emergencies on-site.

Design Symbols:

Maltese Cross: The eight-pointed white cross in the centre is the iconic logo of the Order of St John, representing their long-standing mission of medical service and chivalry.

Green Enamel Border: This green outer ring specifically identifies it as part of the National Dock Labour Board Centre, distinguishing it from general St John Ambulance pins.

Historical Context: These badges were issued to volunteers and staff who successfully passed first aid examinations to serve their fellow workers. The NDLB was eventually abolished in 1989, making items like this a significant piece of British industrial and medical history.