Following on from the last post on the 14th, it seems three German construction firms worked on the tunnel (Information from Michael Ginns MBE).
In 1943 Firma Richert (German) worked on H019, they were then replaced by Hellenbart (Germany) later in that year. In November 1943 there were concerns that the Military Traffic above the tunnel would cause a collapse. The company Theodore Elsche were employed to instal cement lining into the entrance of Ho19.
This is a probably a good time to share the tunnel timeline as we know it:
26/05/1943 Leslie Sinel records in his diary that blasting has started on a tunnel complex under Mount Bingham
18/11/1943 OT requests the closing of the road on Mount Bingham for work to be carried out (lining of entrance)
December 1943 Road is closed for the lining of Ho19, this work can be seen on the RAF photo below.
7/4/1944 Spaniard Luis Nell dies working in Tunnel (memorial stone)
14/05/1944 Electric work is in progress in tunnel C to install lights (Date mark in cement for the light fittings)
The tunnel was still being worked on well after the main removal of slave and forced labours in late 43. We think the tunnel was actively used by the Naval forces after May 44. It seems logical that Ho19 it was used for the storage of equipment as it had been wired for lights, ventilation installed and location markers added in multiple places. It also explains why we are finding traces of equipment in there, like the reel of armoured cable.