Heavy duty 'robust'.
Brass ground rod clamps, aka Acorn clamps.
U.S. manufactured by EMC (Electric Motion Company, Inc. in CT).
Designed for 5/8" OD ground rods, but can also be used with 1/2" ground rods.
The photo shows some of the clamps with a 5/8" OD mast.
$10. for four (4)clamps.