Even more odd

To add to confusion, this is the keycap on an IBM keyboard. This the same key position as a few days ago. The observant will notice that this key has different markings:
Previous: `¬¦
This: `¬|


On the main key press indicates the backtick symbol ("`") and under Linux and Windows that's what you get. So both are correctly marked and it behaves as marked.

The shift-press indicates the not symbol ("¬") and again under both Linux and Windows that's what you get. So again both are correctly marked.

Under Alt-Gr that's where the fun begins. Under Windows I get a broken bar ("¦") which is correct on the black keyboard from last week but incorrect on this white IBM keyboard as it shows a full bar ("|"). Under Linux the situation is inverted, Alt-Gr gives me a plain pipe or full bar ("|") so the white keyboard is correctly marked and black one is wrong...!

Utterly bizarre, especially as I said the broken bar shouldn't exist in the first place...