Read my post above!!!!!! damage to the edge may be from a worn stamping tool, not sure if Clearmount declare they provide a "coated" plate so not sure you can state its faulty.
Secondly, the waterproof properties of leather will ensure any water that does get to the inside of the case does not drain away and there lies your root cause for the rust/corrosion damage..trapped water reacting with a small area of bare metal
Like almost all of these minor complaints you need several things to line up and you have been unfortunate, that said no manufacturer can ensure for every eventuality
At least the damage is minor, does not affect functionality and from what you say 90%+ of the time is totally invisible as you use the phone with the case on
I did read your post hence why I asked why you stated "bare metal". Your post yesterday mentioned the manufacturing process and what with many replies between then and now, your answer was out of sync/context. I would like to steer clear of the water ingress theory, I know my phone and case havent got wet so I'm not keen to hypothesise about it.
As always, the damage depends on the context. I always buy a new iPhone direct from Apple each year. Just got the 7 and was getting my 6s plus ready to sell when i saw the marks, resulting in the amount I'll get for it less than normal.