I bought a second hand set last year and had to fit new barrels (keyed them myself). There is only a small spring loaded plate that holds them in place (right at the end of the barrel) and I found a couple of the plastic barrel guides on the latch had burs. As the lock was turned with the key, the bur would push down the plate and the lock would come out with the key. Just dressed them with a craft knife.
I would also not recommended lubing them at all. This could make the plates stick in the down position.
If you have one unlatched, and look up under the mechanism, you will see the locking plate. You may need the key in the barrel to see it move.