There is very little to go on, and looking at RN officers, he could be any one of a number. The entry clearly says CA Smith, with a G? above the A indicating that he could be CG Smith in ADRIC Register. In fact other regisiter has him as CA in the index, and RIC register has the same man (same dob) down as joining at this time, and joining some 6 months later. He is Claude Albert Smith in both entries in RIC regisister and RN
1920 Dec 12. Joined ADRIC with service no 1377. Posted to M Coy. Down as a "Cipher Expert"
1921 Jan 22 . Resigned