Further ruminations on this, the first thing I could find that used this style of Return was the Selectric II -

If you compare it with the original Selectric layout, you can see that it came from them taking a bite out of the Selectric's return key to fit in an extra key along the row above the home row.

Some of IBM's competitors used it, such as DEC on their VT100 terminal and some DECwriter teletypes

Some versions of the Displaywriter used it.

It seems when they came to make the AT layout, they took the above Displaywriter layout and mashed on bits of the XT layout to make it more suitable for PC usage.
