No idea what this is or what the letters might be.
Was your house built on the site of a previous house?
it actually was a street prior to 1954
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”.
What are/were the names of the local water, electricity, gas companies?
Either that or a prototype slime monster trap…
it’s changed over the years. I’ll check what it was back in the 50’s
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?
Is that another cover lid with screws? The letters look like C U B E, though it could be a 6 and an 8 where I think it’s C nd B
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:
It’s Latin.
It says “Don’t dead open inside.”
No harm in spinning the bolts out I wouldn’t think. If it’s sewer maybe it’s covering a conventional looking plug cap.