Perhaps not the most, Kiev-10, 15, 17 and 20 were never tested after all, and Kiev-19 were pre-production models that were later improved
That's Arsenal for ya
Once I had opportunity to take a look at a Kiev 6c that was used on Odesa film studio almost every day since 1974 until ~2006, at least 6 rolls a day, sometimes more than 10. The pressure plate looked as you imagine, like it was polished with a very fine sandpaper for 30+ years, top cover near the cocking lever was "polished" to brass, the silvery coating, whatever it is, was just rubbed off by finger over time. It's owner said that despite 3 CLAs it just refused to keep consistent frame spacing due to wear, but otherwise still worked just fine.