I'm officially declaring cleaning haze off of the rear surfece adjacent to the aperture blades on Canon lenses as a Crap-Shoot. Just cleaned the surface of another 50mm F1.5 lens for a friend. The haze came off, as did part of the coating. The two "Full Success" stories are my present 50mm F1.4 and the 50mm F1.8 Serenar that G'Man now has. The F1.4 had light haze that came off completely, no damage. The F1.8 is the one picked up for $27 as it looked opaque. Cleaned up to perfection. My F1.2 (sold here) had stains in the Canada Balsam, but the surface cleaned up. Chris' F1.2 (see relevent thread) looks like the oil etched the glass and it needs a Polish and Recoating. Essex, which has cleaned two Summarits for me, could not help his lens.
I have 25 Nikkor lenses in various RF mounts, all from the same time period of the Canon lenses. The only one with interior haze was from 1948 and it cleaned up perfectly. Canon either used a more caustic oil, or a softer coating.