Hi,
This looks like a Yale OBB. Yes it does miss the center cap. To find out the combination you need to remove the "curb" by removing the two screw from the metal disk on the inside. You will then see the wheel pack. This video shows a much newer lock, but it explains how these wheels function:
https://www.youtube.com/watch?v=x6L_jQJRQXQ The opening sequence of this lock will probably be the same as the modern locks:
https://www.youtube.com/watch?v=qEYjgOFKR1U Perhaps the lock won't stop you turning the dial, when it is unlocked, but I think it does.
In any case: Before closing the door please check the complete unlocking and locking procedure at least three times.
