Mike Ford notes that 75mm cartridges were offered before 1900:
"There are dimensions in German for both shells and chambers (CIP) going past 3", but I couldn't find any actual shells. I did find 75mm( 3" is 76mm) cases in the 1886 catalog from Deutsche Metallpatronenfabrik, Lorenz. They were aquired by DWM in 1896."

Marc put me on to the Gevelot firm:

http://eng.gurtner.fr/index.php?p=121&art_id=

http://www.csmcspecials.com/Gevelot_Cartridge_Display_p/gevelot.htm

Kind Regards,

Raimey
rse