Adrian wrote
Michael R N Dolbear ) gurgled happily,

For a single this is unlikely but since in your example even an
Z1-6
Travelcard is less than twice a £3.40 fare

Look past Zone 6.

For me, a pair of singles is over £3 cheaper than an ODTC - there
used to
be an LT card in the middle, but that got pulled.

Not understood. I carried on with the OP's example. If you have a
further example let's see the details.

When is 7DTC capping coming in on Oyster?

Probably never, the complexities, such as ODTC capping day by day, were
glanced at in a post on this NG.

--
Mike D