Yep you are correct, there has never been a Metroland to East London service to my knowledge, in the 2000s they did intend for the Metropolitan line to extend to Barking but this required major re-signalling and it was found to be not worth it
When London Transport was first formed it was by no means obvious how services on the sub-surface network should be organised. Various different service patterns were tried over the following few years, including I believe a peak hours only Uxbridge to Barking service via the Met which I think ran from 1939 to 1941.
The timetable change in 1941 was most significant as this was the one that introduced the basic division of services between the various sub-surface line that we can still recognise today.
Since then the only fundamental changes to the division of services between the various lines of any note are:
- Hammersmith & City and East London lines gaining their own separate identities to differentiate them from the Metropolitan line, thus recognising a situation which had by then existed for decades.
- The peak hours only sections of the Metropolitan and Hammersmith & City lines becoming all day services.
- The introduction of the T-cup Circle Line service.
I’ve ignored changes caused by changes to the Underground network itself e.g. withdrawal of all underground services beyond Amersham.
As an aside the current timetable numbering sequence for the District and Metropolitan line working timetables also commenced with that 1941 timetable change.