Navy personnel in the E3 rate who have qualified as Hospital Corpsman or Religious Program Specialists but who have not yet been rated and promoted to Petty Officer Third Class are allowed to wear a single collar grade insignia when serving with United States Marine Corps units. Similar to a CWO in the Navy, the E3 grade insignia is worn on the right collar point of designated shirts and coats, while the rating specialty mark insignia is worn on the left collar point. (Sailors who have yet to qualify for a rating and who are serving with USMC units wear insignia of grade on both collars.)
This insignia is offered in black as well as anodized and oxidized metal versions, but regulations regarding the wear of a single, non-black E3 collar insignia are not clear. Consult with your local commander for wear guidance.