george has more experience with MB than Daimler, himself; so I listen.

I'm not so sure, though, a genuine MB R4 (even though made by someone else) would need an "X" to identify which is up since they would be identified for one specific use already by MB. It's the AC Delco, Harrison, Delphi models (aftermarket OEM) that would need it since they are used in many different applications, both high and low mount, for different auto makers' marques. On the MB unit the channel/"hole" may well be there, just no identifying mark, since they already know which way it's going to be used because of the manifold port position and the specs on the box.

The lubricant is in the gas so the compressor gets its share, maybe even the front bearing gets enough, but...... the A6, R4 H6 incarnation series has undergone many shaft seal issues over the years. Steel/steel, ceramic/steel, caged/uncaged, single lipped, double lipped, heck, even triple lipped. The admonition to run the A/C once a month to "lube the shaft seal". All of this points to, at least an issue, with lube to the shaft seal which they apparantly tried to address with the extra "port" to the seal area. The oil mist would "stick" to the solid surface and tend to go downhill to the seal. I wouldn't tear a system apart to check it, but if given the choice (and mine had an "X" (my old ones didn't, I'm pretty sure, now)), I'd go "X" up, on a new install.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)