Well, I dare say it's either the pilot's door latch on a B-N Islander, or Trislander, or...
Something that looks identical to pilot's door latch on a B-N Islander, or Trislander.
How could I forget... That awful realisation that you've just strapped in, got comfy, stowed the papers (God only knows where), and are ready to commit aviation, when you realise that the door is still latched open and you have to undo the straps etc to shut it.
OH if correct, please.