You can study digital readouts, cross-hair TID coordinates, #'s, etc.... till you're blue in the face. It won't help you "find the old stuff". There is no special sections of the screen (or certain #s, etc...) saved *just* for "the old stuff"

It doesn't work like that. I sure wish it did, and then I'd just edit out all the other #s or portions of the screen, and only dig old stuff, haha
old coins can read all over the scale, depending on what type of coin, what angle in the ground it's sitting at, depth, soil minerals, proximety to iron and/or other targets (that are skewing and/or masking), etc.... The deeper a target is, the less likely you are to get a locked and consistent TID. Experience with the bouncing cursor, and tones, will tell you what it's trying to tell you though.
Your best bet to get old stuff, is not necessarily to study digital read codes, but rather: to go to better hunt sites, where less junk and pesky new stuff is present. Find virgin sites, get into freshly scraped old-town urban demolitions, be the first one to arrive at beach storm erosion, hit the history books and be the first one at old picnic sites, stage stops, virgin cellar holes, or whatever.