O ring inside filter?

Check your manual that came with the filter. It looks like a groove to hold an oring but also this could be a universal part that fits other filters so you need to verify what your manual states. You indicate you have a DE filter, so mine is Pentair and my manifold slides over that stand pipe and it does have an oring but again - you need to verify with your owners manual and loot at the parts list.
 
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