During my visit to Nine Tiles I discovered what this mod actually did. When
John Grant was coding the ROM for the ZX81 he made a tiny mistake in the division
routines that resulted in an error appearing. It was fixed late on a friday
night and this was the finished product - extra ROMs on top of the original
ROM that had enough power to take over when said division routine was called.
This means this particular machine is a pretty early one.