This man signed himself as SCOTT-WALKER and so his records are filed under SCO in the WW1 Service Records (WO 363). However, several of the records show his surname as WALKER, and it is under that name that his birth is recorded (freebmd Q1 1899)):

Donal James Scott WALKER, born 12 January 1899; an Insurance Clerk; home address Southwood Lawn Road, Highgate, London; mother Jeannie. Enlisted September 1916, discharged 1919. Reg No 514804.

Anc****y, SCO, pages 54215 to 54242