I have certain chemically milled (Acid dipped) parts that I had milled myself and there are obvious signs of the chemically milled process.
Plus, usual other adverse signs of Acid bath is the corrosion in untreated areas and especially uneven surface matter at the edges and seams unless they were treated and this new treatment is a recent trend way after the 60's and 70's.
The treatment in recent acid bathing practices is the application of special coatings to areas not to be milled. This process creates a continental shelf of sought or drop edge between the acid area and neutral zone. These fenders do not have that signature anywhere, but are thinner by a large contrast. I ask because a friend said that they were originally 18 thousands thickness and my measurements are 26 or so thousands. Maybe he was meaning 26 thousands which equates to more or less .18 gauge IIRC.
I've also learned that gauge or thousands readings are different between many metals like stainless, galvanized and regular steel.
I'm confused at the moment to be honest.