It refers to COLUMBO examining the ring on Bremmer's finger. It. felt the diamond ring and figured out it caused the wound to the face of the victim..
" lends a hand" has a double meaning
I never made the contact lens connection. Perhaps “Death Lens A Hand” would be a better title. I never even made the connection with the hand of the title being a reference to Brimmer’s ring. I thought the hand was referencing strangling. But now I remember Brimmer gave Mrs. Kennicut a backhand slap, he didn’t strangle her. So his hand knocked her out and his ring cut her face. Not one of the better titles. Maybe “Through the Lens, Darkly” or “Left-handed Murder”.