Never worked on one of those, and the picture is not all that clear. But, it looks familiar. If its the type I think, there is a spring loaded half ring in the side of the lock cylinder that runs in a groove in the housing.
Some of those locks are "one way". You push the ring into the side of the cylinder, push it in, and it locks, and there is no way to get it out.
I have run into them before. To get them out, you measure in the correct distance and carefully drill in until you hit the groove.
Then just press in the ring and pull the cylinder out.
If you drill in the right place, the ring is only under the hole when its unlocked so that you have to have the key to get it out.
Yours might have an access hole for removing the lock. Look at the side of the housing while you rotate the lock. Look for a narrow brass piece that moves under a hole.
If you see it, press it in and pull on the lock.