Wow!
This is a copy of a Yale OC-5 including the relock mechanism. It is most probably the first Satoh lock, because it is a true friction fence lock with both gears and no spring. My Satoh Sun 700 has still the gear on the drive cam, but the gear on the fence is gone and instead there is a spring. I have also seen locks where both gears were gone. It is quite nice that Satoh has these clips on the cover screws. Never seen that anywhere else.
On the cover plate are three wheels. To change the combination you need to remove them. For this you need to remove the round clip. Something pokey like a stiff needle or a thin nail should do the trick. These clips are quite easy to remove. When you remove piece after piece (wheels and washers) keep track of their order. Maybe put them in a line on the table. The wheels are probably numbered to help reassembly. Each wheel consists of two separable parts, the outer ring with the numbers and the inner ring. On the inner ring is one notch that is important to you. On the visible wheel there is the notch at 73. This wheel is the last number in your combination. The middle wheel is the middle number and the wheel next to the cover the first number. So the procedure to change the combo is to separate the two parts by pressing with your thumbs on the inner ring on the un-numbered side, then move the notch to the desired number and press the inner ring back into the outer ring (obviously from the numbered side). Then put back the parts onto the cover post, finally the clip, Then screw the cover onto the lock. To open the lock you need to dial reverse to the normal order. Normal order is right/clockwise for the final unlock. With this lock nothing will happen. You need to turn it left/counter-clockwise. You can try this without the wheel pack. So to open you need to dial (at least) 4 times right to the first number, (exactly) 3 times left to the second, 2 times right to the third and then left to open.
