I can only real talk about my observations on copper or bronze pennies, as that is my major interest . I find that over stamped letters/ numbers are extremely common on coins up to about 1863 though some can still be seen through to the 20th century . The last I think being 1945. The ones that are of interest to me and I would guess a lot of collectors are the ones that are dramatically out of place or triple struck . Some examples below
Triple struck Y quite sort after , note the G and D overstruck but of little interest
Here's Y over Y dramatically out of place
8 over 8
And just look at this one !!