Navy enlisted personnel in the E2 are authorized to wear collar grade insignia only in pairs except for Hospital Corpsmen or Religious Program (RP) Specialists serving with United States Marine Corps units who have completed "A" school but not yet been rated. In those cases, they are given the option of wearing the USMC or Navy Service Uniform, wearing the insignia of grade on the right collar point and the rate specialty mark—a caduceus for Corpsman, and the Rose/Compass/Globe/Anchor emblem for RP Specialist—on the left collar point. (The Dental Technician insignia, with a capital letter “D” superimposed on the caduceus, became obsolete when that Navy rating was merged into Hospital Corpsman).
Collar grade insignia and branch of service insignia worn while serving with USMC units must be black. Neither Navy or USMC regulations are clear under what circumstances a single anodized or oxidized metal (non-black) E2 collar insignia might be worn. Consult your local commander for guidance.