Vintage Right Test Christmas light bulb tester. Allows you to test replaceable string light bulbs, any size, without removing them from the socket. And, I imagine, you could also test bulbs in a pre-lit tree. There is a sheet of additional instructions inside the package. Uses LR44 batteries. Two were included but as this is old, I suspect it needs new ones; I happen to have a whole box of that type and will include some for you. 

Ships in a padded envelope via your choice of service. Whenever possible, I ship same or next day, so that you can receive your item as quickly as possible.