If I recall from my reading about these old trucks, the original 3 point factory belts had two buckles. The shoulder strap did not retract, I do not believe. The shoulder and lap belt each had a buckle.
some aftermarket options:
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on my '65 c10, I used a 3 point kit from Juliano's for the bench seat:
http://www.julianos.com--
I have read about other folks using a kit from this place with good results:
http://gemstreetrods.com--
-W