Regarding your first question on how to tell a reproduction tag from a factory one, it's an unwritten rule on most forums that this information not be discussed for a fairly obvious reason. We don't want the people reproducing these things to get better at it!
As for your second question, you are quite correct. In the early 80's no one even knew what X codes were, or 4P 3L, etc... on Trim Tags. Therefore why go through the trouble of reproducing a tag to fake a car. However, tags were being reproduced for a competely different reason. In the mid 80's there was a guy that used to advertise in Hemmings that would make a new tag for your car so you could change the interior or exterior color. This was directed mostly to the Corvette guys as they were the target at the time, but he could make tags for Tri-5 Chevy's as well. I owned a 57 Chevy Bel Air in the mid 80's and was thinking about changing the color from the original white (BOOOORING) to something else. At the time, changing colors was kind of frowned upon since it now made the car "modified". At the time, the "Classic Chevy Club" (THE club for tri-5 Chevy's), deducted points for changing interior or exterior colors. I believe the Corvette groups also did the same. Therefore, if one could change colors and still show the correct code on the Trim Tag, no points would be deducted.
Times have changed since then and changing tags to match your "new" color has gone by the wayside! Now people are changing tags to bilk unsuspecting buyers. It's become BIG business and there have been equally, BIG problems when these tags are exposed. One guy got a punch in the nose at a Corvette show a few years ago!! So you can see where things can get ugly.
If you have any concerns or the originallity of your tag, post pictures of the VIN and the Trim Tag here or on one of the other Camaro WEB sites and the experts will be able to help you out.