The opening post said
"installed between the bottom of a meter socket and the top of a disconnect."
That being the case the nipple contains service entrance conductors, grounding bushings are required regardless of KOs left or not.
250.92(A)(1) and (2)
250.92(B)(1) and (4)
From the 2002 Handbook regarding
Note that method (4) requires other similar devices, such as bonding-type locknuts or bushings. Standard locknuts or sealing locknuts are not acceptable as the “sole means” for bonding on the line side of service equipment.