GRCW took the existing Cross-Country design (the Swindon Class 120s) and put a standard Derby design on the front. Externally, other differences included the 119s had more roof vents that the 120s, the guttering at the gangway ends of the vehicles was a distinct angle, and the underframe layouts differed.
A photo by Nick Lander inside a set in the 1970's, still with original interior.
Inside, the biggest difference was in the use of dark timber edging strips for the green fleck formica panels used inside and the lack of curtains for 2nd class, where cloth blinds were instead used. This gave quite a dark and austere interior design.