It was a truth of which I had for some time been conscious that a figure with a good deal of frontage was, as one might say, almost never a public institution.
In the upper sentence, I don't know why the writer used 'of which' to give information about a' truuth'. I can't separate this sentence into two sentences like the follwing.
This is the book of which the cover is yellow.
= This is the book + The cover of it is yellow.
I had been conscious of a truth for some time.