ARP Portal K??? (K1A tunnel - north) [????- ]
The marker shows the approximate location of the northern portal to tunnel K1A, based on the map and a visit.
If you go down the lane behind the Yau Ma Tei fire station, you'll see this CLP sub station at the end:
On its right there's a gate:
And peering through the gate there's a metal door:
It looks like it could be the tunnel portal, but the hillside here has been dug away since the 1940s, so I'm not sure.