I was looking at locks online, and I see a type of lock called a “fission lock,” but I can’t find a definition, nor an explanation of what makes it different from a regular handle lock.
Never seen one in person but a quick google search helped.
Fission seems to be a brand. It is a small mortise lock with a profile cylinder. It is a much more common lockset in Europe than here in the US. If you order one, the most difficult part is going to be installing it. Not many doors in the states are predrilled for that lock footprint.
It looks to be a similar design as you would find on some patio doors. The keyway looks to be down right unusual and I doubt you could find an east way of getting extra keys made.
I might send for a catalog to find out more about them.
Hope that helps.
Osip