If you mean the ones between the flying wires running between the interplane struts like on a biplane, most (if not all) of the ones I've seen have the wires running through the ends of them.
The spacer body is like a round tube (don't know if it's actually a tube or solid, but a tube would be lighter), with slots in the end for the wires to pass through.
Then there is a bullet shaped cap that is fitted over the fore and aft ends to fair the tube. The cap may be held in place by a rivet, or a set screw.
Here's a good shot of the spacers on a very nice R/C model of a P-26. Looks like they run on top of the wires:
http://www.rhc.ro/galerie/view.php?photo_id=1251&screen=1&cat_id=59&action=foto Couldn't find a good snap of the ones on a full scale aircaft.
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