Japanese Tunnel at start of Black's Link [c.1942- ]

With @Mike B and Lawrence on the very start of Blacks Link (where the main road joins the access/country park road), there is a Japanese tunnel off to the right. It goes right into the concrete slope via a small gap that has opened up through the cement somehow. The opening is small but we squeezed in. It opened up and had a number of bends and even a small set of stairs. @Lawrence noted it had a few splits towards the end and I believe we estimated the length as around >40 meters. I have a low quality video but I think we will try to map it out soon. Interesting placement along the same area as all the other tunnels/emplacements on Black's Link, it must really have been well fortified.

Interesting note is that there was an old looking engraving in the mid section of the tunnel; "R.O.G" it seems to say? I attached the photo if anyone has any guesses.