The Bosch 0.280.140.509 (928.606.161.00) is used in 928 until MY 1987.
The Bosch 0.280.140.515 (928.606.161.01) is used starting 1987 and later S4/GT/GTS

There is an actual difference regarding interchangeability. The '85-'86 use a 3-wire ISV while the '87-'95 use a 2-wire one. Big difference. I suppose the price difference also has something to do with the LH version. Lots of cars have LH 2.2 like the earlier 928, but only the 928 has LH 2.3.

It is possible to replace the Idle Stab without removing the intake. Seems not so hard at all. Although I've never done this.

regards
Theo