Raw Ca II K density images from the Arcetri (Ar), Kodaikanal (Ko), McMath-Hulbert (MM), Meudon (MD), Mitaka (Mi), Mt Wilson (MW), Schauinsland (Sc), and Wendelstein (WS) observatories (Sc and WS are shown in a single column due to their low number of images). Each row shows observations from the different archives taken on roughly the same day, except for MD1. The dates of the observations are (from top to bottom): 09 December 1959 (17 November 1893 for MD), 01 March 1968 (29 February 1968 for MD), 01 August 1968 (30 July 1968 for MD and WS), 05 September 1968 (06 September 1968 for WS), and 18 March 1969 (17 March 1969 for MD and WS). All images are normalised to the range of values within the disc and are compensated for ephemeris.