It's a Regulation Police model if it has square butt grips.
Green Frog nailed it, likely from 1952-53.
If it has a leaf hammer spring it's a Post war transistional Model I frame.
If it has a coil hammer spring and 1/2 round front sight, it's an improved model I frame, type 1.
If it has a barrel rib and ramp front sight, it's an improved model I frame, type 2.