In 1948 a non bulk proofed version of the _GY_ was placed into service.
The vehicles were coded _IY_. It seems a decision was made to recode these vehicles as there was already a rivetted _IY_ in service. The new code adopted was 'IO'.
Wagons IO 15870 - 15878, 15893, 15895 were relettered from the new IY group during August/September of 1948. Wagons IO 15896 - 15957 were built new and placed into service between July and November of 1948.
Apparently due to reporting problems [ "10" (ten) instead of "IO" ], a second relettering was done to the wagons. The code _HY_ was adopted. Wagons issued after 26/11/1948 were all HY. The recoding to HY of previous IY/IO wagons of this type took place during the last months of 1948.