There are reports and physical remains of what is believed to be a metal gate that could be dropped across the southern end of Parsons Lodge Tunnel. It would appear that this was constructed in WW2 to seal the fate in the event of an enemy landing on Picnic Bay.
The exact nature of this portcullis is not know, but it would appear to have been a gate suspended on pulliyes. It could be rested on large iron hooks above the tunnel mouth. There were T bars across top of the mouth of the tunnel to allow the gate to be held here when open. It could then be dropped down on to the road when required.
Thanks to Andrew S for some of these images and lots of information.