As 10ringo10 says, there should be two spring loaded latches, one above and one below the main central bolt, which would engage and keep the door closed if the door was pushed shut.
There is a picture of the bolt work of a very similar Frans Schmidt safe here, it is the second safe shown :
http://www.moessongallery.com/safes--brandkast.htmlIf the method suggested above using a filed down key does not work, I suggest side-drilling a small hole for each of the two spring loaded bolts, so you can push them in to gain entry. I am loathe to suggest drilling, but I think here it may be necessary & I get the impression that it is a common technique used to deal with this issue on such safes as well as similar mechanisms used in antique strongboxes. Repairing and disguising the holes afterwards should be easy. Working out where to place the holes may be the biggest challenge unless somebody can provide measurements.
The Frans Schmidt safe in the link I posted isn't identical to the one you are dealing with, but if it uses the same lock, it should be possible to find the drill points by taking measurements relative to the keyhole from the photo & re-scaling them to the safe.
Wish I could be of more assistance, hope this is of some value.
...Mark