This is a Uncle Mike's Holster known as the Inside Pocket, model 8744-3.
These are ambidextrous inside the pocket holsters. It is an open top holster providing grip up positioning and preventing the movement of levers" buttons or catches that are common to loose pocket carry.
Features:
Non-slip band retains holster in pocket when gun is drawn
Gun rides upright for a clean draw and is protected from lint and debris inside pocket
Size 3
Fits 5-shot revolvers
Sigma .380