Fair warning that I did not install or tinker with this latch. It seems to have been repaired a couple of times, but prior to me. In my mind's eye, I see Martin, the previous owner, up here with his ball peen hammer, sinking in a little silver nail to help the latch latch.
This is called a Blake style thumb latch, which according to Ed Donaldson Hardware Restorations,
were manufactured by most of the major hardware manufacturers for many years. Patented in 1842 -- this type of thumb latch could still be found in some form in catalogs for about 100 years.
In any case, without more ado, here is the outside of one of the bedroom doors upstairs ...