I’ve seen enough sewer caps but the photo below is the first ‘Clean Out’ cap I’ve ever seen. It’s a good thing.
Among the objects I seek out when walking are manhole covers and other iron plates that cover accessways to public utilities. I’ve encountered many interesting sewer covers, manhole covers, grates, etc. I don’t know what the plate below was protecting, but I do know who made it. I love the lettering that identifies the foundry.