#1 - Pneumatic-Tube Room General Post Office London 1897
Photo: Cassel & Co., Lim.

#2 - Central Hall Postoffice London
Source: Magazine The Engineer Dec. 18, 1891

#3 - Descriptive Sketch Pneumatic Tube System London
Descriptive sketch of the transit of telegraphic messages by pneumatic tube system from district offices to the Central Station.
Source: Magazine The Engineer Dec. 18, 1891

#4 - Map Pneumatic Mail London 1891
Diagram illustrating the pneumatic tube system of London, 1891
Source: Magazine The Engineer Dec. 18, 1891

#5 - Air Pump Pneumatic Post London
Section of an air pump, with connections to the mains.
Source: Magazine The Engineer Dec. 18, 1891

#6 - Air distribution system Pneumatic Mail London
Mains and pipe connections in Central Hall.
Source: Magazine The Engineer Dec. 18, 1891

"The Engineer" magazine - source of some of the images above - still exists, check their website at https://www.theengineer.co.uk. Because of the age of the images the copyright has expired and they are believed to be in the Public Domain.