W123 Axle Spacers...Are they flat or not?

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Regarding the axle spacer that goes on the axle inner joint, right where it goes in the the differential.....

Are the spacers flat and can go on any way, or do they have an inner and outer side to them?

If they have an inner side and an outer side, how do you tell which is which?

I did not see a reference to this in my searches, and my dealer did not know. I will be installing a new one in the upcoming weeks and want to put it on the correct way.

Thanks