Look at state of it; not for much longer by the looks of it. I wonder why the other end looks so clear in comparison. (I thought you said the original tube had been smashed?)

It had been. This is an unknown tube. Unsure of the age no etch or nothing to work with. The original was half there and half across the pavement with the original sleeve generously pulped by the rain. Looks great though. Presume it's just a standard 535 by the looks of it.

Love it when they get like that...

Only T12 I own where this has happened. What causes it?

Dunno, high engery electronic bouning off the filament and blasting the phosphor away?

An unlit picture featuring the cap and pins & we may be able to identify it...

Extreme heat has risen here? Stuck starter? Some of my t5s have done this mildly where phosphor has flaked off on the inside and you can see a blue glow.

It maybe the positioning of the cathode assembly on manufacturing, does it look closer to this side of the tube than the other?

Over the years I have seen this on plenty of unshielded T12 tubes. Sometimes the 'window' is round like this but it is often a tear drop shape and seems to form when as Angryhorse states, the cathode assembly is quite close to one side of the tube. On a couple of occasions I have seen a small piece of cathode extend from the crimp on the glass envelope side. This piece of cathode is sometimes only a few millimeters away from the glass and sometimes manages to become the source of the hot spot which makes this effect happen quite rapidly and very noticeable.