That is a piece of scrap aluminium, from a storm door.
It is pivited on a small nail,it has a small notch [ covered by lid ] towards the end which closes on to a small nail head which you can see, to lock it in place, also a small nail head it leans against when open.
I keep it open most all the time, except if I am feeding from the top, or have placed honey frames to be cleaned out, this prevents robbers from a close entrance.
Blowing rain I don't believe is a problem, the slot hole is I believe 5/16 by 1 1/2 in. the box is 3/4 in thick so I think very little water would be blown onto the frames of the hive, it would run down the inner hive box wall then out the bottom entrance.
I should post some better pictures of my homemade equipt/junk.
Thanks for the comment.