Let's take a look at the U.S. NAVY FLAMEPROOF GENERAL PURPOSE KEY XXX-26003A:(38KB) This is the widely used navy key with the "Navy Knob", a doorknob style knob with flat base under it. Made for the navy by several manufacturers whose code letters precede the model number. For instance, CJB-26003A was made by J. H. Bunnell. The key pictured in video, CTE-26003A was made by Telephonics. CMK-26003A was made by McElroy, etc. It was apparently copied from the German Luftwaffe Key of very similar design. The Navy Flame Proof Telegraph key, also known as the Flame-Proof Key (FPK), has a history rooted in naval communication during the early to mid-20th century. This specialized telegraph key was designed to be used in environments where the risk of explosions and fires was high, such as aboard naval ships. The key was constructed with materials and features that minimized the risk of sparking, which could potentially ignite flammable gases or materials present on a ship. The design typically included flame-proof insulation and enclosed contacts to prevent sparks. This was crucial for maintaining communication systems in hazardous conditions, especially during wartime. During World War II, the Flame-Proof Telegraph key became an essential tool for naval communication, allowing operators to send Morse code messages securely without the added danger of igniting explosive substances. These keys were used in conjunction with shipboard radio and telegraph equipment.