MIG wrote:

In another thread, someone posted the result of an FOI request, which
included a TfL staff briefing, which included this statement.

"An Oyster Extension Permit is only needed when a journey is started
within the zones covered by a Travelcard at a LU/London Overground or
DLR station and involves travel by National Rail beyond its validity."

What happens at the joint stations like Stratford or Richmond? Is there a
consistent rule or does the passenger have to know how the station has been
designated?