Yes, that is normal. You should see 3 to 4 threads extending through the arm rods enabling you to securely attach the arms and tighten the thumbscrews without fear of over tightening or stripping the threads. If you find it difficult to extend the spine mounting bolts through the curved arms, mount the curved arms to the spine after placing the fabric on the arms but before attaching the end tension rods between the arms.
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