you are also going to find that not only is the date code cryptic, but the contents are going to be closely protected by 99% of the main kit manufacturers. All of them have very proprietary processes to get the grape concentrate, and the formulations on the kits is usually in flux to create the same results everytime.
The only kits you get that you can be reasonably sure of Harvest date and general location are the RJS EP Hightail line, which are only available near RJS in Canada. These kits are made to be "hightailed" to market, so are usually only out for a few months the from the years harvest.
Other kits that specifically state appelations (Wahaipo valley Sauv Blanc, Delu Woodbridge, etc) will have to source most of the product from that area, but due to the nature of how kits work, we will have no idea of the age or other details of the harvest.
In order to truly get that information, one has to forgo kits and move on to fresh grapes that one has sourced. Of course that requires crushing, pressing, and testing for various levels of acids, free SO2, chromatography tests, etc. Usually, you'll make much larger quantities with fresh grapes because of the labour involved. 6 Gallons takes just as much effort as 60.