David Jackman wrote:

But outboundary travelcards should never be issued from within the zones,
and I wasn't previously aware that National Rail stations could issue Z1-9
either.

The extremities of the Metropolitan Line are a special case when it
comes to the "out-boundary" rule, in that National Rail ticket offices
*can* issue a Travelcard for them (Amersham to All Zones being the usual
scenario) *and* they don't turn into a pumpkin when you reach Amersham.

It also appears that the rules regarding other out-boundary Travelcards
have now been updated in "The Manual" and it now explicitly states that,
whilst not valid for another journey back to/from London after you
return to the origin point, they *do* retain the Travelcard validity
within Zones 1 to 6.

Cheers,

Barry