There are many advantages to using aluminum. I am going to throw out my theory on this one. Bauxite mining in the US peaked in 1943 (Bauxite is the ore for Aluminium). due to the aerospace war effort. I'm assuming after the war the supply heavily outweighed the demand causing aluminum to have competitive pricing against steel. Also the war effort most likely accelerated the use of aluminum in more common applications. The aluminum fenders are lighter so they should be cheaper to ship and easier to install.
The real reason for these I think comes from clues in the letter as using these as 'service replacements'. These are large pieces with alot of shape. That means large dies and alot of surface wear. Service replacement parts are ran on the production dies after the production run. Dies wear. If you need to supply X amount of service replacement fenders on already worn dies this is a very good way to do it.
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Redline Design
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