This structure was entirely destroyed when the bunker was built, as described by workers who were drafted in to do the work during the war.
The only remaining part was the levelled initial section of the passage, about 30 m south of the dolmen IIIa portal. The initial section is drystone walling, although the workers remember slabs. So the structure was probably semi-megalithic, like grave IIIa. It is also likely that both crypts had similar dimensions: like chamber IIIa, chamber IIIb was probably built against the cairn II facade.