In those days there was the Mercury 9.9.
This type had the same cilinderblock an displacement as the 6 & 8hp type (198cc).
When Mercury started to use the cilinderblock of the 15hp type (264cc) for the 10hp model instead, they liked to communicate this difference to the market in a proper way, so they called it 10 "XR". This gave the engine the sportive image it needed, being about 12hp.
Hope this little history lesson got made you a good answer
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Regards, Wouter