OK, you have an older lock where the combination is set by hand rather
than with a key. The vertical bar on the top right of the lock and
spring mechanism in your photo is a relocking device. If someone were
to forcibly punch in the dial in a burglary attempt, it would be pushed
back and release the spring-driven bar into the notch in the boltwork
preventing opening. When you take the back off of the lock, you'll need
to reset it to its present configuration when reassembling.
If you remove the two screws and the back cover you can see the inner
workings which are a series of notched wheels and a lever with a bar
called a "fence". It's best to have a second person turn the dial while
you observe the mechanism. As you spin the dial you'll observe how each
wheel has a pin (called a "fly") that engages the next wheel. Turn the
dial several times until all wheels are moving. When the notch (called
a "gate") in the last wheel is directly under the fence, note the
number on the dial. Change direction of rotation until the gate of the
next wheel is directly under the fence. You'll probably need to turn
two full turns and a partial turn. Note this number. Change rotation
again until the notch in the last wheel is under the fence. Note this
number. Then turn until the fence drops into the gate and keep turning
until the bolt slides back. That's your combination.
If you wind up with the last movement of the dial going the wrong way,
start all over but begin turning in the other direction.
There are several YouTube videos that go into some detail should you
want to change the combination. Here's one:
https://www.youtube.com/watch?v=x6L_jQJRQXQ Google "hand change safe
combination lock" for others.
Note that once you figure it out and have a working combination,
reassemble everything and test it at least TEN TIMES with the door open
before you lock the safe. It is FAR easier to fix things with the door
open than with it closed.