Single Knuckle Duster SFK 02 Stamp Monster Anodizing Type 2

Dimensions:
Material - Aerospace Aluminum
Coating - Type II Monster Decorative Anodizing
Height - 1 15/16" (49 mm) 
Width - 1 15/32" (37 mm) 
Thickness - 43/64" (17 mm) 
Finger hole diameter - 1" (25 mm) 
Weight - 1.9 oz (54 grams)

More similar products you can find here:
cool aluminum 1 finger edc knucks buy in the USA

Special hole allow you to use this product as a Keychain.
You can also thread a 550-type paracord lanyard through the hole.
Nice gift for Him.

EDC Craft Store official website is the best place to buy original EDC-products at manufacturer's price.