This is Drawing 2550 in the SR original locomotive drawings collection in box 131 roll 490 (there are other speed indicator drawings in this roll and other drawings for the Class G). To view the list see http://www.nrm.org.uk/~/media/Files/NRM/PDF/archiveslists2012/railwaycompanyworks/Southern%20Railway%20drawing%20list%20Works%20Composite.pdf
