The only thing I found was this.
It looks like it's some designation of how the chips are installed on the PCB. It's either single sided, or single chips on each side. If you can, take a picture of the ram chips (one of each, the stock and the crucial) and we can look at them to verify if that is indeed what the S stands for. I can't imagine that it would cause an issue because that article says a mix is what servers should run. Run just the crucial and see if that ram posts and works and all that, and then that should answer your question of if it's a compatibility issue.
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