With the Canon LTM Lenses: the later lenses in Black, vs chrome, used a newer type of high-refractive index/low dispersion glass. It was susceptible to damage from the oil on each side of the aperture outgassing. Most of the fog can be cleaned off, but there can be damage to the coating, and even etching to the glass. I've seen a lot of that. With that stated- the lenses are great, the Canon 50/1.4 is the reason why Leica came out with the improved Summilux. Be sure to have an inspection period for the Canon lenses.
I have a lot of Canon LTM lenses- 5cm f2, 50/1.9, 50/1.8, 50/1.5, 50/1.4, 85/1.5, 100/4, 100/3.5, 100/2, 135/4, 135/3.5. (notice how I did not start the post with this...) They are all clean, all good. The 100/3.5 was badly fogged, a forum friend sent me two of them to use for parts after he was told they were so damaged that it was not worth opening up to give an estimate. They cleaned up far better than I had hoped, I sent him back the one he had bought 40 years ago and kept the other. BUT- I've also had Canon lenses with etched glass and permanent "Finger prints" in the front surface. The 85/1.5 is very under-rated, I got it for $400. Try to find a Leica or Nikkor 85/1.5 for that. The other modestly priced LTM lenses are the old Minolta line.
