If we know something about the mass distribution of the lens then, for a given cosmological model, we can predict the lens redshift and magnitude from the image separation. I did this for a poster at the IAU Symposium 173: The Astrophysics of Gravitational Lensing in July 1995 in Melbourne: for those lens systems with unknown lens redshifts and for some representative cosmological models.
The links below point to one file containing the complete text and a separate file containing the figures, not individually but as included figures in LaTeX---6 pages with several small figures, caption etc. on each page (one page for each lens system).
As are most of my publications this is available in two forms: