The manifolds may be different. The driving factor is the interface between the manifold and the compressor.

Your compressor has the same model number as the one I installed. The milled-out holes on the back of the compressor have different depths and different diameters.

The W123 manifold in my car (and I assume most W123's) had a flat surface with a short section of tubing. The yellow plus the larger diameter hole and shallow depth, plus the green and the smaller hole and deeper depth combined for a flat surface a mm or so above the surface of the compressor.

When you tighten this combination of parts down, then you're compressing the rubber in the washers for a tight seal.

I assume it would also be possible to get close by using the red and the green, and this would create a bit more stand-off from the compressor.

Not all R4's are the same. Older ones used O-rings instead of sealing washers, as described above, my Harrison used two same-size black ones.

I would suggest that if you don't use the yellow one, then you 'might' need the little section of tubing in the kit to keep the sealing washer centered in the port since the milled out diameter of the larger hole is a few mm larger than the red or green. Maybe vstech will chime in there.

I'd definitely dry-fit before committing to the install with oil in the compressor and being upside down on your back. It sucks to be in that position trying to figure out why things don't fit together...

Thanks YAK--Tray Carlisle recommends the Yellow and Green for the 1980 300SD. Appreciate your input.