Dixon flexible keys
The end of the shaft is placed against the keyway and then the head is pushed down the shaft to feed the links into the keyway, then it's turned.
Here's a picture of the lock itself.
When we got our hands on the lock, several of the links of the key were broken, so we had to jimmy it in the most Mexican way possible: we flattened the caps of two Corona bottles, trimmed them a bit, and shimmed either side of the padlock.