Single Finger Brass Knuckles SFK 01 Stamp Blue Anodizing Type 2

Dimensions:
Material - Aerospace Aluminum
Coating - Type II Blue Decorative Anodizing
Height - 2 3/64’’ (52 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:
real aluminum one finger edc knucks price 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 official website is the best place to buy original EDC-products at manufacturer's price.