These two pennies are both dated 1968.

The one on the left is a normal penny. The one on the right is the chickless .

The principal difference is that the chickless penny has the body of the second chick missing in the area behind the hen's leg. The head of the chick is quite clear to the left of the hen's leg. but the curve of the chick's belly and the top of its leg are completely missing from the areas to the right of the hen's leg. There is a tiny remnant of the chick's leg visible just above the exergue line.

There are other differences between the coins, for example the chicken's far wing - visible behind the raised foot - is smaller on the chickless variety. It is possible that these difference are the result of an over polished die, however the variety does occur for other years. 1942 examples are also available and I have a report of one from 1948                    

Error chickless