I'm a busy boy these days but I seem to recall a memo in which these rods were explained. As I recall Belin was out at the Paine house with some members of the DPD, when he asked Mrs. Paine if she'd actually had any curtain rods. When she told him yes, and showed him the rods, he decided it was best if they checked the rods to see if there was any evidence Oswald had handled them.
That's they way I recall it, anyhow. If my recollection is accurate, well, then, that would explain why the DPD knew the exhibit numbers.
I hope somebody with more time clears this up.