The Sunilla tool is one of the oldest through the lock tools. Like many fire service tools, this tool was invented by modifying another tool. Capt Sunilla of the FDNY ground out the groove in the end of a standard lock puller to allow it to pull rim lock cylinders.
Rim lock cylinders are only held in place by two screws and a small metal plate. Older rim lock cylinders had a small bridge on the top that the modified nail puller would capture to pull the cylinder. This tool was easily and quickly adopted by criminals which caused lock companies to change their cylinders.
To combat the Sunilla tool, lock manufacturers started making the face of the lock of very soft metal so the face would break away when trying to pull the cylinder. Also, some lock manufacturers began making the cylinder round to prevent the Sunilla tool from biting onto the cylinder. Round lock cylinders were the reason for the invention of the K tool.
This tool has been made obsolete by modern lock cylinders but was cutting edge in its day. We can thank Capt Sunilla for modern through the lock tools as they have all evolved from his firehouse ingenuity.