Yes it is. The book points out it's rare to have a 1968 closure in reasonable nick like that, and also that there's a big rock holding the lid shut.
We have 3 round here in East Lothian, all 1991 closures, one of which is unlocked, but now has a good foot or more of water down it, and the hand pump is seized. Shame.
Most of the 1968 victims seem to have been "demolished" however I suspect the mot common method was just to collapse the vent and access hatch turrets, chuck the rubble down the hole (shut the door at the bottom of the ladder first) then dig them over. You can still see two mounds where Eyemouth's post was, for instance. I can't imagine it having been cost effective to completely remove them.