I've used regular marathons for ages on my commuter bikes and both these photos show significant tread wear. Heck I have some old 26x1.5 marathons with about 10,000 kms on them and look new compared to the wear on this tire.
The second photo with the wacky wavey wear is very significant, I've never seen that before, and as suggested, could be from chronic under inflation.
Pretty tough tires to still be in one piece though.
Idea--i wonder if being 20inch tires the added rotational speed was a factor?
I really do try keep the tires at 60 psi - most of the time - what do you recommend? I have a hand pump and an 18v Ryobi. Keep in mind - there are many years and many miles on these 20" tires. Thanks for the info.