Bewick:
I believe that Guy Motors in pre Big J days used the DB box behind the Gardner engined Invincibles and the Turner 5 speed behind the 7:7 AEC engine Warriors. I think that once Guy Motors via Jaguar ownership joined the Leyland Group they started using the Thorneycroft 6 speed box as well as the Eaton Fuller 610 particularly behind the ■■■■■■■ 205/220. I stand to be corrected but Thorneycroft had originally been taken over by AEC so I assume it was their “in house” axles and gearboxes were the cheaper alternative. I stand to be corrected although all six of the Big J’s we ran at Bewick Transport all had the 180LXB and the “Thorney” 6 speed box. Cheers Bewick.
It was the Thorneycroft / AEC D203 6-speed gearbox, used in Mandators, Mammoth Majors, Marshal Majors, also used behind the Leyland O.680 in the Scammell Handyman. And probably something else I can’t remember now. You are quite correct it was BL group policy to use components from within the group if possible. They did have range change and splitter multi-ratio versions of the Thorneycroft / AEC “D” series 'boxes but they weren’t beefy enough for the torque of the big ■■■■■■■ engines.
And a bit of useless information, I was looking something up today and came across some publicity for the DB 6-speed 'box. It was available from 1962, I hadn’t realized it was as early as that.