With any safe of this type access to the lock can be gained by removing the back lock pan. The pans are secured in a number of different ways, but they all involve some sort of screw or bolt. It was the fashion to fill the slotted heads of the screws and paint over them so that they were no longer clearly visible.
A word of warning. When the manufacturer put in countersunk screws, they would often grind the head to get it flush with the surface. This can leave a very sharp edge on the screw head. So be careful.
I have been castigated on the site for the next bit of advice. The screws are generally different lengths and the heads are dissimilar. If you want to keep it all original, then mark where each screw belongs and put them back in the correct holes. If you are not bothered about it remaining original, tap them out to the closest metric size and put some new bolts in. I had a large stock of allen head countersunk screws and used to use those, simply because I had them, but they do do a very good job.
