Diesel Water Separators / Pre-Filters - Difference between these?

Happy to stand corrected, but R U sure?
The last time I checked a Racor installation manual, I'm sure that the diagram only showed the tank side on the right and the engine side on the left (looking at the filter), without mentioning any possibility of rotating the housing.
Whose front and rear aren't symmetrical, as I recall - even if the main body is round.

Yes, have done it. Only thing that bugged me was the sticker was then on the wrong side, shame they didn’t supply sticker for you to attach yourself after installation
 
Now to answer the OP's question.

Water separators are just that, they are more commonly called agglerometers and their sole objective is to remove water from diesel, a water separator can mean anything used to remove water from anything and a misnomer.

Pre-filters is also a misleading term as they can be used for different purposes, in marine applications it is usually to remove the solid sediment picked up from the fuel tank, so we can reasonable conclude that they are the first line of defence in removing solids from the fuel lines before it hits the main filters.
It can also be a combined filtration unit which includes the agglerometer and solids filtration unit in a common housing and essentially pre cleans the fuel before it hits the main filters.
 
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