His Military Pension Ledger Index Card gives his service numbers as follows
R. D. C 81946,
D. L. I 11721,
R D C 66162,
R Guernsey L I 2346,
AMPC 13025453
The last one AMPC (Auxiliary Military Pioneer Corps) suggests that he was still serving in the army well after WW1 and possibly into WW2, which might explain why you cannot find him on the 1939 Reg.