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A 'Berntsen'.
Cross section

Berntsen is an American manufacturer of survey equipment. A number of the passive stations installed by the OS, in particular the FBM auxilaries, used a survey marker from Berntsen.

Details from the manufacturer available here.

The trig mark / datum point is located under the lid and is typically a 5cm domed disc. However, a simpler, centre punched rod has sometimes been used, as noted below.

Several Surface Block Passive Stations [1] have Berntsens which have been discovered within a few metres of them. It is unknown whether they were installed by the Ordnance Survey, but as far as is known, none of these Berntsens have their own Station Number. Examples include:-

Failand [2] (marker is a 15mm centre-punched rod)

Bailbrook [3]

Moor Fields [4]

Ballards Ash [5] (marker is a 15mm centre-punched rod)