This is a tubular key from Southco, used for their specialized E3 Series Large Size panel latches. These are often used with locks on RV's, tent trailers and sometimes tonneau covers in pickups.
These keys are really sturdy and last a long time... unless they somehow get lost.
Note: These keys look a LOT like the Southco E5-26-819-15 but they are not interchangeable! Using the wrong key in a Southco lock could damage the tumblers in the lock and cause irreparable damage. If you're not sure which lock you have, or which key you need, please call us or email us, and we'll gladly help you determine the right part number.