Thank you for the pointer, I am somewhat confused by the name of Keates' head voicer you mentioned - "Herr Otto". Would "Otto" be his first name, perhaps? There was a Swiss organ builder named "Johann Andreas Otto" who, however, didn't seem to have left Switzerland. I was under the impression that the connection between Edmund Schulze and Keates was a man named "Karl Schulze" {not related to E. Schulze, only employed by him). It looks like there must have been more Germans working for Keates then...
Neither the tierce mixture, nor the "Harmonics" mixture are common at all in our neck of the woods (Central Europe, south-German organ building tradition), especially if it is to be the only mixture in the organ. But it seems to me that the English tradition (at least as represented by Keates) seemed to have preferred the Tierce mixtures over quint mixtures - that is if there was to be just one mixture stop in the organ. Would this be true for other English builders of the time as well?