I have several LTM 135 lenses that I've picked up "for comparison purposes" (yeah, sure, snort). I have two Canon 135 f3.5's in chrome (but not marked "Serenar"), one Canon 135 f3.5 in black, one Kyoei Super-Acall 135 f3.5 and one Leica Hektor 135 f4.5.
I took them out on my balcony, screwed my Bessa R onto a tripod, and tried a variety of shots, from wide-open to closed down tight at infinity. Should have shot some short and medium-range shots as well, but there you go, I forgot.
I had a very hard time telling the negs apart - except for the Hektor. I was surprised.
I had bought the Hektor to use on a Zorki 4K before I got the Bessa R (and the Zorki fell apart; literally). The Hektor would not mount on my Zorki anyway, the focus tab snags the rangefinder foot on the Zorki and knocks it out of alignment. I paid, as I recall, $45 for the Hektor on eBay - it has some light scratches on the front element. All in all, I had not been impressed with it, and put it back in it's plastic tube and ignored it.
However, after testing...
The pictures from the Hektor were VERY different from the pictures from the Canon's and Kyoei on the same roll of film. First of all, it was better wide-open then the others. Of course, wide-open on the Hektor is 4.5, not 3.5. Second, the colors were more intense, and contrast was, for lack of a better word, creamier. After scanning the negs at 2800 dpi on the Minolta Dual Scan III, the black Canon 135 was the clear winner for 'sharpness' or 'acuity' in terms of objects being identifiable after severe blow-up. But I really liked the overall look of the Hektor MUCH better!
The Hektor is much lighter in weight than the Kyoei or the WOW heavy chrome Canons, but it is a bit longer.
I'm sorry I don't have the test images to post, but I'm in Baltimore right now. I plan to do more tests as soon as I can.
I suspect that the Hektor is non-coated, but perhaps I'm wrong. In any case, I sure like it better than the Canons for the character of the color and the overall feeling it gives.
Sorry to be so subjective, but just my 2 cents...
Best Regards,
Bill Mattocks