It's part of the extrusion. You could mount an aftermarket track on top. try a search for the other thread there was alot of info in it.
Todd
"I said, now, I said ,pay attention boy!"
The cure for anything is salt water - sweat, tears, or the sea
Isak Dinesen
If a man is to be obsessed by something.... I suppose a boat is as good as anything... perhaps a bit better than most.
E. B. White
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