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.
The 1985 – 1986 32 valve S3 idle speed regulator (only on US Cars and the few European 32 valve S3s with the no-lead fuel option) is quite different, it has three wires and is much smaller. The S3s require idle speed and CO% base adjustment to work properly.