Okay. You said it was like ten inches down, so I'd guess it's probably not something related to your house, it's probably some bit of infrastructure from the old street. Which means we should be thinking in the realm of "outside municipal access covers", and not "something installed to make my house better".
Can you try that thing where you put a sheet of paper over the letters and gently rub a pencil sideways over it, see if you get any more details on the letters?
Not sure it helps, but I fiddled with the levels & stuff to try to make the lettering clearer. Didn’t have much success, but I do see something that could be ‘co’ to the right: