Jefferson Nickel type I & II

What is the differance between the 1939 nickel, type I & type II coin. I thought I knew collecting untill I began looking at types, D/S, D/D, 3 legged. Has there ever been a type III?
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There are several varieties depending on the die pairings.
I do know that there are two types of 1942 Jefferson and 1913 Buffalo nickels though.
My first post...updated with pics
I collect mostly moderns and I'm currently working on a US type set.
Reverse of 1938 has a thinner top step while the reverse of 1940 has a thicker top step.
-Amanda
I'm a YN working on a type set!
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Unless you are dealing with high end mint state coins with full or nearly full steps, it does not matter much.
When I saw the title I was expecting a question on the two 1942 types.