What is the purpose of this slot?

The shaft of the wick raiser slides over into it so you can close the door. Some lamp styles have a sliding retainer inside that holds the shaft in place so it doesn't slip back out and block the door as you're closing it.

And my Hiram Piper (Canadian) version has the latch to keep the wick raiser out of trouble..........mld

Thank you. These didn't come with the burner, but I knew there was a good reason for it. They fit the brackets on my caboose! We're going to refurbish the caboose this year.