4ontheFloor,
If you are referring to non-slotted valve covers with 4 hold-down scews/bolts, they could be 1955 or newer heads and/or valve covers on a non-PVC engine.
The 1960 261 (non-PVC equipped) had a road-draft tube, a non-slotted valve cover, and a vented oil filler cap.
What size and year are your engines (blocks and heads)?
Tim
