Charles Ellicott Commentary Numbers 4:6

"and shall put thereon a covering of sealskin, and shall spread over it a cloth all of blue, and shall put in the staves thereof." — Numbers 4:6 (ASV)

And shall put in the staves thereof. —The staves had been removed while the Ark was being covered; otherwise they remained in their places (Exodus 25:15). (Compare to 2 Chronicles 5:9.)