In message op.uyjjm50pby8eno@sheepdog, Colin McKenzie
writes

The replacement for 73 stock is already out for tender.

That's a bit early, isn't it? If the modernisation programme is over
budget, deferring this build would be one way to spread the cost.

IIRC the 73 stock went into service in 1975, so its replacements
shouldn't be needed until 2015.

It's (currently!) designated 13 Tube Stock so that's about right.
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