These are small little knives that fit on your finger, and they are used to quickly cut butchers twine and casings through production. They come in
Twine [Ring] Knife
Ring Knife, size 12, 12/pk (priced per case) – Pursell Manufacturing
Shop – Page 3 – Craft Butchers' Pantry
Ring hole knife : r/knives
PLYS-Outdoor kitchen knives Hand forged high hardness steel knife tools Forged razor knives - AliExpress
RING KNIFE, (12/BOX) - Grower's Nursery Supply
Victorinox 5.2533.21 8 Bread Knife with Slant Tip - Black Fibrox - Globe Equipment Company
Handy Twine Knife Razor's Edge Metal Ring Knife, Straight Blade, Size 14, Box of 12 - KnifeCenter - HT04
Victorinox Victorinox Swiss Army Knives 53951 Cavalier - Red
Handy Twine Knife Original Metal Ring Knife, Curved Blade, Size 14, Box of 12 - KnifeCenter - HT03
Cutlery-Pro Santoku Knife, 7in
Magnesium Blade. Multi-Use. This twine/ring knife from Zenport features a sharp magnesium blade and can be worn on a finger for cutting twine, rope, packaging, tape, cardboard and box cartons. Zenport Style Number RK.
Zenport RK6 Ring and Twine Knife
The Original Handy Safety Knife - 12-Pack - Utility Ring Knife for Finger with Sharp, Curved Blade - Ring Size 9 - Yellow - Standard Blade - by Handy Twine Knife
Handy 2-pk Twine Knife Ring Size 10 Curved Blade Utility Finger
Handy 2-pk Twine Knife Ring Size 10 Curved Blade Utility Finger Razor String Box Thread Cutter