In message , Clive
writes

In message ,
writes

I wasn't clear what the Moorgate numbering scheme, especially as these
things change over the years. What are now the FCC platforms were the GNC
and not Metropolitan ones at the outset.

Has this any connection to the Northern City Line? As was.

Yes: originally the Great Northern and City Railway, opened in 1904.
Purchased by the Metropolitan Railway in 1913, transferred to London
Underground in 1933, lost its potential moment of glory when the
Northern Heights project for the Northern line was abandoned after the
war, cut back from Finsbury Park to Drayton Park in 1964 to allow for
the Victoria line, renamed as the Highbury branch of the Northern line
in 1970, transferred to British Rail who made a new connection to the
surface lines at Finsbury Park in the 1970s and now part of the FCC
network.

Although seemingly much unloved over the years, it's now back to
fulfilling its original intended purpose of easing congestion at King's
Cross by running commuter services down to a more convenient city
station at Moorgate.
--
Paul Terry