I can see it with the naked eye. It looks just like a hard water stain.
If it were a grinder or welding slag, only a small part of the glass would be affected as well as the metal around it, not all the windows evenly. At least, I don't think so.

One evening this week I'll try to use some 0000 steel wool or maybe a razor blade to see where it gets me.

Phil Forrest

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1972 220D "Trudy," named by a friend.

"The 220D sounds good... I suspect it is the only car that you need a calendar for, rather than a stopwatch, when doing acceleration tests."
Tom Abrahamsson