6.5 US Navy – Shellback Challenge Coin Release The Kraken 13 of 40. Navy Shellback Challenge Coin is a rare and valuable collectible, highly sought after by militaria enthusiasts. Measuring an impressive 6.5″ x 4.3″, this coin stands out with its large size and detailed craftsmanship. It features a military theme, emblematic of the rich traditions and pride of the U. The Shellback designation is a symbol of crossing the equator, a significant milestone for Navy personnel, making this coin not only a piece of history but also a symbol of honor and achievement. Ideal for collectors of current militaria, this coin is a unique and distinguished addition to any collection. Important Notes (Please Read). This is a new open box item. Pictures of the actual item are attached.