My 1965 parts book shows two different part numbers on the 65 fenders.
Part numbers specific to the A990 cars, and part numbers for regular fenders. So to me that would make sense to have been made from different thickness sheet metal.
The regular part numbers are 2525856 and 2525857 for plymouth.
A990 plymouth fenders 2416885 2416886.
Dodge regular fenders are 2525858 and 2525859
Dodge A990 fenders are RT 2416888 LT 2416887.
The A990 part numbers are also backed up in Jim Schilds 1965 Dodge and Plymouth Super stock Authenticity guide.
Also from Jim's book and I am quoting
"the doors were identical for dodge and plymouth.
The super stock door shells were lightweight steel and carried part numbers 2416889 for the right and 2416890 for the left. Most era literature states that these doors and the other lightweight body panels were built and then acid-dipped or chemically milled to remove metal thickness. Chrysler Corporation Product Planning,Technical Section, Fleet Engineering Detail Instructions (A-105,ARAP 5030, dated September 22,1964) indicate that these parts were to be stamped from 0.018-inch thickness steel which would have required modifications to the stamping process as the presses are designed for a specific metal thickness. It is possible that the lightweight parts were made both ways depending upon when they were manufactured but existing lightweight body panels were measured and they are 0.024-inch thickand the parts did appear to have been dipped."
Other details for the lightweight sheet metal is the LT door has no mounting holes for an outside mirror.
The front fenders will only have holes for the coronet badge. The right side part number 2416888 will also have the standard small gold pentastar emblem. There should be no radio antenna hole that might normally be found on a stock fender.
Matt