The spacer/collar you have goes against the clip on its wider face. Its inner face forms the stabilising wall for the outermost o-ring. This ring is stabilised on its other side by the missing spacer. This in turn provides the stabilising wall for the innermost o-ring. That's stabilised on the other side by the rebate in the bore of the rail connector. The spring clip compresses the o-rings so they are a snug fit on all four 'sides', radially and axially. Without this arrangement, the o-rings can shimmy sideways and fail to seal under pressure. The use of a third o-ring as a substitute spacer isn't providing the same sealing surface since the rings in contact with each other have soft surfaces. It's better than nothing as evidenced by the number of rails using three rings, but it's not anywhere near as reliable as the correct setup.
TC
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