Well it's not really a manhole cover because it's much smaller. But it
doesn't seem to have another name.
Anyway, after 40 years it had rusted in place.
I tried a hacksaw blade around the edges. That worked OK down to about
half an inch. But the cover is 3 inches thick down to the ledge.
Tried a 4 foot crowbar in the hole, levering in all directions.
Tried hitting it a hundred times with a sledge hammer.
Tried heating it very hot with a blowtorch.
Tried freezing it.

Then I made a 16mm key to fit in the hole, with a ring on top.
Tried levering it up with a 6 foot crowbar but the crowbar bent.

Eventually, put a 5 ton jack under the crowbar, and the cover lifted
up.
I estimate that I applied about a 1 ton force with the jack, and the
4:1 leverage of the crowbar made it about a 4 ton pull to lift the
cover.
Here is the cover just starting to lift:
http://i38.tinypic.com/28i6j4m.jpg

I've ground the edges of the cover with an angle grinder and put CRC
on it!