What is the history of a 1975 memorial penny inscribed with a map and liberty bell on obverse side?

Private novelty companies place these marks on ordinary coins and sell them as "valuable collectibles." When the buyers try to get the implied profit after a few years, they find that a coin dealer will give them only a fraction of what they paid (if they will even buy them) and the coins get spent. If you have one I haven't seen, I would probably pay a dollar for it. (Sorry, I already have that one.)