The center pin extends from the end of the main body about a half an inch.
The sleeve barrel screws on from the cable side and is not allowed to come off, but when engaged with the SO-239 female part the sleeve roughly matches the position of the center pin.

So the barrel end will start flush with the major base area of the pin, then when connected, the barrel is screwed on to the socket to hold it in place.

Go here:
http://www.rfcoaxconnectors.com/Technical_Interface-drawing.htm

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