Does anyone know what this is above the boiler flue?

Hello everyone, this is a Thatcher boiler. I think it is a Thatcher red king oil boiler that was converted to gas. The house was built in 1940 and the model number on the boiler says 056. I have been trying to purchase a manual or book to help me learn more about the boiler but no luck since the company closed in 1968. We purchased the house in 1988. I need to learn how to do a treatment flush, because my Dad used to do it and I used to help him sometimes but now I forgot how to do it. I just remember it was not that difficult, but I have brainfog from grief. He passed away recently. Any info is much appreciated! Thanks!