At the co-ordinates
You're looking at the top of a stone column and it's capital. I see this every day on my walk to the tube station and always wondered where it was from and why it was there...
Classical Orders
Architectural Orders are the ancient styles of classical architecture, each distinguished by its proportions and characteristic profiles and details, and most readily recognizable by the type of column employed. Three ancient orders of architecture—the Doric, Ionic, and Corinthian—originated in Greece. To these the Romans added, in practice if not in name, the Tuscan, which they made simpler than Doric, and the Composite, which was more ornamental than the Corinthian.
What you can see is the capital (top piece) of a column of the Ionic order. It is easily recognisable by the volutes which are the spiral, scroll-like features at the top. This order originated in the mid-6th century BC in Ionia, Greece. Look at the main entrance of the British Museum and you will see the same capitals.
Where it is from?
The capital before you is all that remains from the General Post Office Building which was demolished in 1912. It wasn't even in Walthamstow but was located on the eastern side of St Martin's-le-Grand, just to the north of St Paul's Cathedral. It was presented by Messrs Mortimer Ltd to Walthamstow Urban District Council. Originally it was displayed in Lloyd Park from 1913-1954 before it was moved to its current location.
The cache
Look at the column and count the number of flutes (vertical curve profiled grooves about 1.5 metres in height). It's a two digit number. This number is AB.
Replace A and B with the relevant numbers and this will take you directly to the cache:
N51 35.0(B-1)(A+3) W000 00.9(B+2)(Ax2)
