Which preposition to use with improvers

of Occurrences 13%

Bales was the improver of Dr Bright's system, and, according to his own account in his "Writing Schoolmaster," he was able to keep pace with a moderate speaker.

from Occurrences 1%

I am desirous to add that this attention may be useful by preventing improvers from running into every specious scheme of agriculture produced by a lively imagination and engaging them to study the great variety of soils and even climates in this island, and to be careful in adapting to these their several operations."

on Occurrences 1%

Are these erudite persons prepared to insist that the name "Father" should no longer have any sanctity for us, because in their view of likelihood our Aryan ancestors were mere improvers on a state of things in which nobody knew his own father?

Which preposition to use with  improvers