In message , Tim Roll-Pickering
writes

Yes but with travelcard seasons on Oyster one has never needed an extension
ticket.

Never? How would one have got to (say) Barnes in Zone 3, with only a
Zones 1-2 travelcard on an Oyster before this year?

Someone with a zones 2-4 could enter at Stratford and leave at
Liverpool Street with a straightforward extension deducted.

You are using a particular (and quite unusual) example of historic
Oyster inter-availability to make a general point that didn't apply to
the overwhelming majority of NR passengers in London.

Forget the special case of Stratford to Liverpool Street and consider
how a Zone 2-4 travelcard could have been extended to Waterloo at this
time last year (bearing in mind that there were no Oyster readers at
Waterloo NR and that PAYG was not accepted by SWT). The same applies to
London terminii used by other TOCs, except in the few cases where there
was already an agreement to accept Oyster PAYG.

--
Paul Terry