Eric has nailed it. A fairly common European practice. One of the arms is attached to an L shaped "hook" that grabs a [removable]brace across the middle of the smokebox opening. The other arm is internally threaded and serves to adjust for a tight seal. Notice there are non of the dozen or more "clips" and nuts more commonly seen in American smokebox door practice. I'll see if I have a piicture of one of our German engines opened up, which will make it more sootily apparent.
John Harbeck
Hesston Steam Museum in Indiana