The hobo nickel, an early form of creative currency modification, had its heyday during the circulation run of the Buffalo nickel, from 1913 to 1938. Since they were small, cheap and easier to carve, they were popular with hobos, who often used them to trade for food and goods. (Jefferson nickels and other denominations were used, but the old "Indian Head" design was considered best because the large profile gave the artists a greater area to work with and allowed for finer detail.)
The period from 1940 to the late '70s saw the Buffalo nickel almost completely fade from circulation and with this, the styles took a decided turn to the modern. In the early 1980s, there was a resurgence of hobo nickel carving and collecting and this time period marks the separation between the "old" and "modern" eras.
You can find many examples, old and new, at The Original Hobo Nickel Society.
Wikipedia provides a serviceable survey of the topic.
thanks to Ledgergermane for tip!