They show up in several WWII era pubs, (TM 3-300)? Seems to ring a bell. Issued empty, filled in the field from 1-ton containers. There was a pre-made charge as mentioned by highlandotter, but generally the plan was just several wraps of detcord on the wires on the back. While identified as a mine, it was not. The concept was similar to a minefield, deny/focus entry or degrade performance while passing through. You would take the M1 and function it (and others) causing an area of persistent contamination that would last for days. Enemy troops would then be forced to avoid it or deal with it (protective clothing, decon, etc). Since they were never filled there are plenty of the M1s around. They used to release them for purchase at PDO sales as oil cans, you had to buy them by the case.