You're reading listings from goofy people that don't know how to "attribute" aka list the applications for...parts.
The reason some say (except mega cab) is because for MANY suspension parts, the 1500 is actually a 2500 series truck. soooo...the people from the data company, that ride their bicycles to work, Came up with a ram 1500 application that states (ram 1500...except the mega cabs) They are blindly posting this on fender as the "application" even though it doesnt matter because it's a body part.
you may even see...1500 (including mega cab) and 1500 (excluding mega cab) in the same listing.
2006-2008 ram 1500's are the same fenders. a 2009 2500/3500 remained the old body for one year longer, so if fits that too. Some vendors find the lesser of evils is to list LESS applications...than trying to explain to novice consumers the difference in a 2009 ram 1500 fender...and a 2009 2500 fender because its not worth the return shipping.
so the info in post #2 is correct. the last 2 numbers of a mopar PN are just revision codes. pretty irrelevant. i'd probably look at Sherman or LKQ before i got hammered at a chrysler dealership.