Holborn was opened by Great Northern, Piccadilly & Brompton Railway on 15 December 1906 fitted with a 19 lever frame. The box was closed on 14 July 1979 but the B frame may at some stage have been replaced by an 11 lever unknown frame.
Holborn signal box was located in the station building. It was opened by London Transport in 1933 and was fitted with a 11 lever Westinghouse 'N' frame (frame number 176) which was replaced by a panel on 2st March 1994.
The frame shown there is that which was located on the Central line and the siding was located between Holborn and Tottenham Court Road on the site of the closed British Museum station
Photograph shows the interior of the signal cabin on Platform 6 at Holborn station, the picture was taken in July 1993, Picture by kind permission of © Nick Catford
There was however another signal cabin on the Aldwych shuttle platform at Holborn which controlled access to the Aldwych branch from the Eastbound Piccadilly line. Concensious appear to state that this frame had 7 levers. My thanks to Steve Roverts for this information.