The Walthamstow Fragment – an Ionic Conundrum

Outside Vestry House Museum in Walthamstow Village there sits a short column with an Ionic capital.
Its irregular shape kept me intrigued for quite some time because I noticed that no two corners are the same – one corner is a true right angle, the next an acute angle, the opposite one has a corner cut into it and the last is an obtruse angle. Oh, and for some reason the one true corner is angled at the very top edge.
And, rather than all four side faces being the same, giving the illusion of a scroll falling on a column when viewed from any angle, this one depicts a scroll cut and flipped through the diagonal, such that faces A and B are alike with the usual egg and dart design, but faces B and C have some central foliage.
It's rather hard to explain so here are some photos:

It's an architectural conundrum; a stone mason's nightmare.
Intrigued, and thinking it was an art installation, I looked for an information plaque, but could not find one.
Having just researched it online, all I can find is that it is a relic of a GPO building demolished in 1912. But it doesn't say which GPO building or where exactly it stood.

. . . . .

I wrote the above and hit 'save' rather than 'publish' and went to check out porticos with ionic columns. And I now see that it's all very simple...
This Walthamstow fragment must have been the left corner of a portico. For example, see St Pancras Church opposite Euston Station – for the corner columns of the adjacent outward-facing sides to both depict a scroll they must have a common protruding angled corner piece, and the opposite internal corner needs a chunk 'missing' so that it aligns with the flat scrolls on the interior sides of the other columns (are you following this at the back?!).
Blimey, that was hard work, but I do now feel enlightened.
As for the irregular obtruse and acute angles I noticed on the Walthamstow fragment, I am now wondering if I didn't see that at all; the mind/eye can often play funny tricks... Or maybe, if I observed it correctly, the GPO building was, indeed, a bit out of true or built on a strangely-shaped plot of land.
A return visit is needed, or if anyone lives nearby, perhaps they could go and have a look for me and report back.