In message of Tue, 9 Jun
2015 12:43:38 in uk.transport.london, Recliner
writes

[snip]

In any case, there's currently a surplus of S7s, as Derby has produced
more than the District line yet needs -- most of the D78s are still in
service.
I found this thread interesting, but frustrating.
The statement above is typically frustrating.
I assume it is using some facts, written in a URL, somewhere.
I would appreciate links.
--
Walter Briscoe