Individual and extreme coins below - first - what actually happens to make a dryer coin
My Grandfather had laundromats for years.
He found many "dryer" coins b/c he serviced his own machines. First is a group photo of one batch - could not locate the others right now for a larger group photo.
BTW - a common mistake I see concerning these coins is that people think the coins tumble around inside the tub where the clothes are. Since the clothes would cushion the battering of the coins rim, and certainly a person would not repeatedly, accidentally put the same coin back into the dryer hundreds of times to accomplish a battered rim, this does not make sense. So what really happens?
Commercial machines had an inner (clothes) tub, and an outer tub that surrounded the inner.
Coins would slip out of clothing being put into the machine and fall into the skinny opening (at the "mouth" of the machine") between the two tubs and be tumbled through many cycles.
On a front-loading machine, after awhile the coin's thickness could increase enough such that the coin would be rolling on its edge between the two tubs rather than continually being carried upward and dropping back down.
Edited to add three notes:
1. When I say "top loading" on a washing machine, this actually refers to his (possibly old-fashioned now) washing machines that had angled fronts on them.
2. To retain scientific accuracy, which demands observation (by definition - despite modern trends), it needs to be said this is still theory
as to how these dryer/washer coins were/are made since there was/is no way to observe the process. Remember the space between tubs was/is hidden when the machine is assembled! Until someone puts a transparent front on one of these machines and witnesses the process, it remains a theory.
3. Actually, the description of the process is a not simplistic as above since the inner tub of a dryer has indented areas in its circumference to help the clothes inside tumble (meaning a cavity on its outside into which a coin can fall); the inner tub of washers had perforations in them that had nubs sticking out in between the gap in the tubs; and also the back of all inner tubs, I believe, have a small space in which the coins might also sometimes enter (?).