Hello,the gilkes turbine will always have a cast number sitting proud on the spiral casing.Once you have it give Gilkes a ring and ask for Wilf Curwen,they keep fantastic records of every machine sold and Wilf will be thrilled to answer your questions.
Do you mind me asking in which area this machine is,hydro power is my business and i have just refurbished a very similar spiral cased francis for the Highlands.I reckon this looks to be one of thier 1920/30 designs.
regards peter.