From page 11 of the Whistler WS1080 owner's manual (same scanner as the Radio Shack PRO-668) this is how you install the swivel belt clip:
"Swivel clip allows the scanner to move
with you and easily move it out of the
way without unclipping it from your
belt. To attach the Swivel clip to the
scanner; align the belt clip grooves to
the slots on the back of the scanner.
Slide the clip down until it clicks in
place. To remove the belt clip, pull
back on the small tab and slide the clip
upward."
The full owner's manual for the Whistler WS1080 can be downloaded here:
https://cdn.shopify.com/s/files/1/1420/2702/files/WS1088_Manual_12D15.pdf