British Standard 497 still refers to them as "Manhole Covers"
They were given that name because a man went down into the "Manhole", or "Inspection Chamber", or "intercepting Chamber", or "Silt Pit", to perform any necessary procedure.
A "Lamphole Cover" was a cover for a much smaller chamber, big enough to allow an oil-lamp to be lowered on a rope into the sewer, to confirm visually, (from upsteam and downstream), that the sewer was not blocked.
Or you could perform a smoke test with a smoke bomb; or use drain rods with a badger, and ... ...
A-a-a-h! Happy, happy days.
No, I was not an Engineer --- I was a Quantity Surveyor, but I was a bit of an expert on Drainage --- a sort of "Connais-Sewer", (as the French say).
Les