Gilles, Great scans ! What kind of scanner are you using?
4th pic looks like a plate scratch, 5th pic - guide lines, 6th pic plate scratch. Not sure about 1 and 2 but could be a re-entry. The 20c has alot of minor re-entries.
Here is plate position 28 from re-entries.com. I think there is some similarity. Do you see any marks in POSTAGE? I have a copy of pp28 and it has the same guide dot at the lower left corner.
re-entries.com

jimjung's copy
