WWII 'Arrow Slits'

Just before my camera decided to die on me, I was taking a lovely Autumn walk up by Ranger's House and around the perimeter of the park - I add this picture just as a reminder that it wasn't that long ago that we saw the sun...

As I rounded the South West corner of the park, up near the Tea Hut and the nice old K2 phone box, I saw this in the wall:

Judging from the angle of the holes, I'd put money on it dating back to one of the World Wars - presumably gaps created for rifles to poke through and set their sights on invaders over Blackheath. The cement dressing makes me think they're unlikely to date back much further and be some kind of musket-slit (unless repointed...)

It hadn't occurred to me that it would have been considered a likely Nazi landing spot - after all the heath wasn't quite the smooth billiard table it is now - it had hollows and dips all over the place. But I guess it was a wild area near London - and any walled area was a safe haven to be defended.

According to Neil Rhind the First World War wasn't nearly as devastating as the Second. Zeppelin raids targeting other things claimed one or two direct hits, and it saw the end of golf on the heath (some might argue no big sadness there...) but the worst damage was afterwards, when the detritus of war wasn't cleared up for years and lack of money sent Blackheath into decline.

I'm guessing that the rifle-slits I saw the other day date back to 1939, when Greenwich (and the rest of the country, natch) was a hive of preparation for the Second World War - seeing practice drills by the emergency services, searchlight batteries erected, barrage balloons inflated and trenches dug (which apparently were a bit of a hazard - people kept falling into them during the blackouts...) Neil Rhind says that one gravel pit was used as a home for a special barrage balloon that could be floated into the sky at short notice.

At least 12 V1 and V2 rockets fell on the heath, but I'm not aware of any Nazi parachutists so presumably the elaborate preparations did the trick although I'm sure a bunch of poor sods from the Home Guard spent many a night with their guns stuck through the holes waiting for invaders.

I have a wonderful image of them in my head - the Corporal Jones character, alert and ready to panic at any second. Fraser predicting doom and gloom. Pike wearing three extra scarves his mum's made him and Godfrey just wanting a pee. Private Walker, of course, has snuck off round the corner to the Tea Hut for a roll up and to sell some black market stockings to the girl in orange lippy serving his cuppa.

I'm curious - does anyone know of any other quiet reminders round here of the war that have somehow escaped being cleared away? The odd pillbox? Air raid shelter sign? Anderson Hut in someone's back garden, used as a shed ever since?