French or Belgian. I’ve seen a few setups and they always have a single latch bolt + multi-throw main locking.
The position of the latch bolt can usually be determined by the mark it has left on the door ‘frame’ over the years. The depth of it can be determined easily and then a very small side hole will allow the bolt to be pushed back and the door opened.
Should be easy and take no more than 10 mins all up.